Terry Hughes is waiting for me as I arrive at the hilltop that overlooks the DMZ near Sudong-ri. If the area had looked overgrown and rusty before then it looks even worse now on the Northern side, with young trees pushing up through the dragon's teeth tank defences. Hughes doesn't look like the veteran of the Z-War that he is. Instead he more resembles a university professor, with glasses, a mild expression and rumpled clothes. You wouldn't think to look at him that he has been leading sweep teams throughout the last remaining hot zones of the undead for the past ten years – and the man who has recently returned from the first mission to North Korea since the Z-War. He greets me with a handshake and then he gestures at the DMZ – and then at the hills behind it.

Look at it. Dead as a doornail. And the main reason for the lack of curiosity about what happened there. Miles of defences in depth for some of the most paranoid minds in human history. (He makes a disgusted noise and then sits down on one of the folding chairs he has brought)

Speaking of paranoia, do you have any idea how long it took to get permission from Seoul for just one reconnaissance mission? Five fucking years, that's how long. Two years to do the planning, three years to get permission. We'd submit a plan, they'd tell us it wasn't good enough, it wasn't in the right place, it wasn't at the correct time, we had to wait and come up with a better plan… it almost drove me raving mad. Doing the planning for sweeping the London Underground was child's play in comparison.

Why did it take so long? What kind of concerns did they have?

Well… some of their worries were valid ones. Walking in over the DMZ would have been suicide because of the defences there – and believe me they have a lot of them. Every now and then a mine goes off and you see falling scraps of deer or rabbit – and I've lost count of the number of tripwires that have been spotted. Then there are the automated defences, the missiles that still get activated every so often when a plane comes near the DMZ. Last thing anyone wants is to set anything else off.

So we looked at going in further north. Problem was, where? The North Koreans had defences along the Yalu as well as the DMZ. The Yalu wasn't as heavily fortified, but we still wanted to avoid it. That left the bit in between. Again – where? Flying in might have set off air defences, so that left the coastline. The western coast of North Korea was no good really as Pyongyang's on that side, and no-one wanted to bet money on if Kim Jong-il had been equally paranoid around his capital.

That just left the eastern coastline – and even then we had trouble working out where to land from the sea.

What kind of trouble?

Any recon requires a safe base of operations – somewhere you can sally out from and get back to easily. We wanted a bay that was deep enough far out to allow big ships to approach to launch landing craft and which then shallowed enough that any sea Zs could be seen coming a ways off. We wanted a crag or promentary that could be used to set up a z-proof zone of observation which we could then use for snipers to pick off any Zs that happened to be in the area as we set up the defences for a base. We needed somewhere close to a usable road and we needed that road to lead to one of the suspected tunnel entrances that we'd identified. That's a lot of requirements. We found barely a dozen possible sites – and two of them were too close to the DMZ for comfort.

You'd identified possible tunnel entrances?

The wonders of satellite photos. Problem was that we didn't have any of people going in to them. We had a few places that we suspected before the Z War might have been access points to tunnels, but not that many. We thought that they must have moved their population in at night. No prying eyes that way. That or they watched the weather forecasts and did it when it was overcast.

No, it wasn't easy. We started off looking for the entrances by studying the state of the roads, looking for heavy usage and wear and tear. Problem is that outside of Pyongyang North Korean roads were never in that good a state of repair in the first place. Add on years of neglect, sun, rain, snow, ice, rockslides, subsidence… you get the picture.

Then we got lucky. The teams who reactivated the computers in Langley and the Pentagon, which someone who deserves to be bloody knighted had left on standby reserve power when they were both evacuated, found lots of nice pictures from the time of the evacuation. No-one had seen them before because at the time people had other things to worry about. We did some comparative work and we identified some possible entry points – places where there had obviously been a lot of traffic in the days before North Korea vanished into thin air.

That narrowed our list of landing places down to three. We worked out an operational plan for each one, we refined them, we polished them, we did everything but gold-plate them. And each and every time we submitted them Seoul – and the UN – told us to wait. Wait. Wait. We got so sick of that damn word!

So what happened to change their minds?

I guess it was two things. The appointment of Hyungchol Choi as the head of the Korean CIA meant that we finally had a major voice willing to push for a recon in North Korea. The second thing was the clearing of the Channel Tunnel. That was a lot nastier than a lot of people had thought because of the sheer number of Zs that were down there – far more than people had forecast. Coming at a time when the Japanese were starting to get nervous about what might be lurking in the Seikan Tunnel… well it started to make people just as nervous about just what might be beneath the surface in North Korea.

So we finally got the go-ahead. Hell, that was a rush. We didn't know if they were going to change their minds at any moment, so we just went ahead with it as soon as possible, as soon as all the pieces could be put into place.

You know it wasn't until I finally laid eyes on the coastline of North Korea just over two weeks ago that I finally felt that we were there. And I remember looking at the beach, and the rocky crag that the first commandos were already climbing up, and the hills and mountains on the horizon and… oh Christ, I can't describe the feeling that I had. That icy brush down my spine that I always get when something isn't quite right with the world. (He falls silent for a moment and then shakes his head.) It's a sixth sense I guess.

Not that it lasted that long, as we were too busy working on the base. The minute we landed we were running for beyond the high water line to set things up – get the supplies onshore, put the watchtowers up, erect the walls, run the power cables, get the generator working… all by the numbers and all the time with the knowledge that there was a good chance that we'd be getting some undead company at any time. Every coastline in the world has them somewhere. That was why we sent the guards up that crag first. They had to sweep it just in case and then act as sentries for us – pick off the strays as they wandered towards us and watch out for any packs.

And they were there. Maybe they'd walked up from South Korea, or down from Vladivostok or under the sea from Japan. As we worked we'd hear the occasional sighting report and then there'd be a crack of a gun and then a confirmed kill report. We had to make sure that we didn't rush things when we heard those. Last thing we wanted was to get sloppy from working too fast. About two hours in we had the basics well and truly started when the first of the seabed Zs finally started staggering in through the surf. They must have seen, or felt, or however the fuck they sense things, our ships going overhead and followed us. Luckily there weren't a huge number of them – just this slow stream coming up the beach and getting their heads blown off their shoulders by the snipers.

The thing was though that we were nervous. Recon can never give you 100% of the real picture, especially with trees and other cover like houses in the area, plus the unknown was a big factor in North Korea. Were there any tunnels near the beach? We were pretty sure that there weren't – the ones that we had located were miles inland – but what if?

(His eyes lose their focus for a moment and then he shakes his head)

No we just kept at it. We finished bang on schedule too – the first time that's ever happened. We ate when we could and then half of us stood guard whilst the other half of us slept. And hell did I sleep! That amount of work and tension takes it right out of you. When I got woken up at 3am to swap shifts I heard that we'd had a few more Zs staggering in and even a few crawlers – the ones without legs, or in one case just a head on some shoulders, one arm gone and the other flailing along with a stump of a forearm.

Next day we extended the perimeter to build the vehicle pool and get the mobile laboratories erected, and then we were up and running.

Mobile laboratories? What were those for?

We had orders to take samples of everything for analysis. We had a few odd readings here and there in Japan and South Korea, from the radiation that had been rained out of the atmosphere after the Iran-Pak nuclear FUBAR, so we were told to see if there were any traces in the North. Besides this was a perfect chance to see what had been happening in North Korea. We had so little information for the place. What kind of harvests had they been getting? What kind of fertilizer had they been using? I know that sounds dull, but if they were all underground then what were they eating? Could they grow anything underground? Don't forget that this was the country that suffered from horrific famines in the 1990's. What kind of stored food did they have? Could we even tell?

Anyway, the day after we finished the base – and by now the Zs had dried up completely, which was a good sign – we got the go-ahead from Seoul for the run to the tunnel entrances. Well, I say 'go-ahead', but really it was more of a committee meeting. The guys in Seoul told us about the latest satellite data, plus the weather forecast, we gave them an update on where we were with the base and then we agreed that we'd go the next day.

I slept well that night, even though we had a storm roll in at about midnight. That's the one thing my fiancée envies me so very much, I can fall asleep in times of stress and stay asleep through non-Z related noises. Meant that when I woke up the next morning I was ready to go from the moment I opened my eyes.

How big a unit were you leading to the tunnels?

A reinforced company. We had three more in the base – two to defend it in our absence and the last one to act as a mobile reserve, in case we needed rescuing, plus a platoon of snipers on that crag.

Plus we had Rhinos – the latest armoured personnel carriers, with all the bells and whistles like flechette cannons, heavy machine guns and a bulldozer blade. Top speed of 40 miles an hour. Ugly buggers, but reliable as hell.

We piled into five APCs and left just after 0600. It was a nice day, sun, blue sky. We were doing about ten miles an hour as we headed up the hill to the road – keeping a close eye on anything that moved. Each Rhino had a K-unit with them, with those dogs sniffing the wind and wagging their tails. Heh. (He laughs.) Mine had this daschund-terrier cross. Best dog I've ever worked with. It was his pre-retirement run. That dog's got a list of medals as long as my arm. Best nose ever. Terrible name though. "Mister Boodles" his partner, Katherine, named him. I love that woman – hell, I'm marrying her! – but what a name. We just called him Mr B, and Christ almighty did he earn his pay in North Korea. Sometimes he'd pick up the smell of an approaching Z twenty to thirty seconds before the other dogs. That might not sound like much, but it's a lifetime in combat.

As for the road… Christ it was terrible. Potholes everywhere, leafmould everywhere, sprouting weeds and saplings everywhere… We really blessed the suspension on the Rhinos, they gave us a smooth ride despite everything that road threw at us. We went up it, with the Ks – especially Mr B – sniffing in the wind, and all of us alert to a squirrel coughing.

We'd all seen the satellite pictures of the route, so we knew what it was going to be like, but that didn't mean that much once we were on it. We found one small landslide that hadn't been there a week before, and we had to stop three times to clear fallen trees away. It took two hours to get to the big landslide we'd seen in the pictures and during that time we had about… I think four encounters with roving Zs. There weren't that many of them and one was pretty far gone – must have been one of the earliest victims in China. God knows how he'd gotten as far as Korea, but he was almost falling apart, nothing but frayed tendons and white bones and fragments of skin.

I think the worst part of that trip was waiting for the landslide to be cleared. It was a big one, blocking most of the road. Oh, we could have squeezed past it, but the last thing we wanted was a bottleneck on our line of retreat. The Rhinos went to work with their blades whilst we set up a perimeter and kept a tight watch on the area – and on the slip, just in case it contained any buried Zs.

Once that was clear we barreled down that road. The thing that got me most about it? No streetlights. Hell, no catseyes in the middle of the road – just this strip of tarmac that was going back to nature at a gallop. I think that road said a lot about North Korea.

At about 11.30am we finally approached the area where the tunnels were suspected to be, and the tension level went up a notch… or three. The Rhinos slowed and stopped and we eyeballed the area – god, I sound like a Yank don't I? – through binoculars. The road went around this corner and we could see a hill on the other side. But what stopped us dead at that point was the pair of concrete bunkers on each side of the road. Big concrete bunkers, with three slits on the front of each to give a bloody good field of fire. That made us stop and think. And then we saw the amount of undergrowth around them. Trees, bushes… well, the bunkers didn't look maintained, and that field of fire was getting very cluttered. But still… we didn't know, so I got out with my second in command – Major Kim Yong-Hee, a great bloke, very sharp – and we walked very slowly up to them, stopping every now and then to inspect the approaches. Longest walk of my life. Oh I didn't really think that there was anyone in either of those bunkers – the amount of undergrowth around them meant that no-one could be maintaining them – but… well, we didn't know for sure, and uncertainty of that kind is always bad on your nerves. Besides, even though the Ks were silent, that didn't mean that there might be a Z lurking somewhere. I remember looking through the closest slit on the left-hand bunker with all the caution I could summon. But there was nothing in it – I could see clear through to the open door at the back of it. Kim reported the same with the other one and only then did I relax a little. As I gestured for the others, who'd been covering us from the Rhinos, to come up I heard this thoughtful noise from Kim. I turned and he was standing there with his head tilted to one side, looking around the bend of the road. "Interesting," he said, "Sir, I think we hit the jackpot."

I walked up next to him, stepping over the remains of the wooden pole that must have been marking the entry point, and looked. Jackpot was the fucking word.

It was a big open space. The hill sloped down quite abruptly about 400 yards away, and there, cut into the rock, I could see these five… gaping maws. Tunnels. Big ones too – not small ones, these had to be about twenty feet high and thirty across. The minute I saw them, this… shiver went straight through me, like a shock.

There were just the tunnels there?

No – but that's what we thought at first. As the company started disembarking Sergeant Flaherty came up to me and pointed to the treeline about a hundred yards away. "Is that a building, Sir?" she asked, and I took a look through my binoculars. Sure enough, it was. Single-storey, concrete, trees all around it, with a flat roof that was covered in leaves and even some small saplings here and there. It had a door facing us, but it looked like it was half-hanging off its hinges.

Well, that was our first port of call. I took a platoon with me to check it out whilst Kim was inspecting the rest of the area visually. Mr B was on point and believe me every time he stopped and sniffed we all stopped with him.

Closer we got to the place the worse it looked – the building that is. Cheap concrete construction, trees all around, leaves and dead branches underfoot... the place looked like it was falling apart, under relentless assault from Mother Nature. As we got closer we could see that the door almost falling off – but we still waited for the go-ahead from the Ks before we approached the entrance.

That door almost fell apart as we approached it. The hinges were a mass of rust and the rest of it wasn't much better. It was dark on the other side and using our torches we could just see this corridor, with doorways leading off it. That and water on the floor, along with leaves and dirt.

Taking the door off wasn't hard – but it did generate some noise and we stopped for a moment to let the dogs have a sniff. Nothing. First two doors in must have led to dormitories or something, because we could see marks on the floor by the walls that might have been from metal bunks. Nothing in there though – the rooms had been stripped clean of everything.

It was the same with the next two doors – but then we got to the last one. That one was different. It was padlocked from the outside and was in a better state than the outer one.

(Hughes looks down at the ground for a moment.) We all knew that this looked… wrong. We couldn't say why though. Hairs on the back of the neck territory again. We got Mr B in – and he sniffed the doorway and then he whined a little. That was his way of saying that something was dead in there – not a Z, just dead. We weren't going to take any chances though, so we took the safeties off the guns and got ready as private Slovik stood by with a set of boltcutters. Then, when we were ready, he snapped the padlock off and pulled the door open, before stepping back.

It was… the oddest scene I've ever seen. It still haunts me even now. The room wasn't that large, but there was a table in the middle. Three shattered windows over to one side, with branches and tendrils pushing in. And a body on a chair by the table. He was very dead – had been for many years, since the start of the Z War I'd say. Ordinary corpse, not a Z. Not in very good condition, but we could tell three things about him. The first was that he was dressed in the uniform of a high-ranking member of the North Korean Army. The uniform was rotting and falling to bits, but the hat, with that huge crown, was on the table and was very distinctive. Second thing was that he'd blown a hole in his own skull. There was a rusty gun on the floor and a piece of his skull was by the far wall. And the last thing… was that someone had watched him die. There was a security camera attached to the wall, pointing straight at him. He'd been there in that room, that he'd been locked into, and he'd blown his own brains out on camera. There was a tendril of ivy around the camera that had pulled it partly off its mounting, and it was dead as a doornail, but it had worked once.

(He shakes his head in disbelief) Can you imagine anything like that? I still can't.

Did you find out who he was?

No… oh we have theories. He was too tall to be Kim il-Jong, not that anyone really thought it might have been that stunted little murderer. We still have no idea. Whoever he was though – someone must have hated him. He was deliberately shut out of Kim's underground anti-zombie bunker, out of North Korea's equivalent of a communist sanctuary.

Do you know if he'd been infected or anything?

The body was tested later on. No trace of Solanum at all. He was clean – but why had he been forced to shoot himself? Why was it apparently taped? Who was he in the first place? Not enough information.

Once we finished in the building – we didn't find anything else – I came out and found Kim and Flaherty looking out over the area and talking quietly. Kim told me that they'd spotted a few things and I pulled out my binoculars.

First thing they pointed out was what looked like a cutting off on the far side, opposite the tunnels. It looked like something had been stretched over the top of it, like camouflaged fabric, but it had sunken in many places. Had to be about, oh, 200 yards long.

Second thing they drew my attention was the more important of the two. There were more security cameras overhanging the mouths of the tunnels. They weren't pointing at us, just at the entrances. That got me thinking, and thinking quite hard. For a moment I thought about the sheer level of paranoia, but then I remembered that it was North Korea.

We checked out the cutting first. I stood by with the rest of the company whilst Kim went forwards with two Ks and about ten scouts. They went slowly and carefully, making sure that they were in sight the whole time… all the way over. When they got to the cutting they stopped… and then I could see from their body language they were no longer tense, just puzzled. They walked about a bit for about ten minutes – and I could see one of the dogs scratching its ear, so I could tell that they weren't sniffing any Z's – and then they called me over.

When I got there I found Kim squatting on his haunches, with his helmet perched on the back oh his head, looking down at the cutting. I could see that it was about 15 yards across and had been covered with what looked like a series of camouflage nets. Of course after all those years of rain, and snow, and sun and leaves and animals and so on, the netting was looking very much the worse for wear, with great holes in it. I think the only reason why the satellite pictures hadn't seen it was the fact that the trees were so close to one side – you'd be amazed at what shadows and worn camouflage can hide.

And the cutting was full of Soviet-style lorries. Sorry, trucks. There must have been fifty or sixty of them, neatly parked. They'd never go anywhere, their tyres were all flat, there was rust everywhere, the engines must have been falling out them, their windscreens were thick with dirt when they weren't cracked or shattered. But they were parked so neatly that they looked as if someone had left them there expecting to use them again quite soon. It was eerie. Kim told me that as far as they could tell the trucks all had full tanks of petrol.

So someone must have been expecting that they'd be used again?

Yes, exactly. Which got me thinking that perhaps the North Korean evacuation to the tunnel system might have been intended as a temporary one. That or they were just really anal about their transport. It was just a passing thought, that first theory, but it really highlighted how much we didn't know about what had happened there.

That turned my attention back to the tunnels. The cameras worried me. I thought that there wasn't much chance that they were still working, but that wasn't a theory I wanted to gamble on. So we started looking at the far right tunnel. It was closest to the entrance and I could see from my binoculars that the camera was pointing down more than the others, so if it was on it was showing a lot less of the outside.

I got Majewski on the case. Polish corporal, very smart, very observant and he could climb like a human fly. We kept him covered from the Rhinos as we watched him climb up what looked like a sheer fucking face of rock and then he scrambled about and over to the front of the tunnel. He peered at the back of the camera and then he looked at something next to it and then he turned to look at me, pointed at the camera and then drew his finger in a line across his throat, meaning that it was dead.

As he climbed down I took a platoon over to meet him next to the entrance, along with Mr B. "Very dead, sir," Majewski told me as we approached. "The case is waterlogged and the power leads have been worn by friction on the rocks. There's no chance of it having worked for years."

I nodded and then I looked at the tunnel. (He shakes his head) To tell you the truth, I did not want to go into that thing. There was… something about it. I guess it was nerves. Well, after a moment of thought I got half the platoon guarding the entrance and then I started leading the other half down the tunnel in a standard swipe-and-wipe formation. Two scouts first, then a K, then the main body led by me, then the reserve.

Well, it was dark in there. Ten yards in and we had to clip our torches to our guns and turn them on. What the light revealed wasn't that great when it came to North Korean workmanship. The tunnel was damp underfoot, with moss and lichen near the entrance. The walls looked like poured concrete – and not hugely good quality concrete come to that. We saw at least one set of tree roots starting to peek through the roof. And then, about twenty yards in the scouts' torches revealed something up ahead. This… gleam of metal. We all pointed our torches down the tunnel and could see this… huge door at the end, about 50 yards away. It filled the tunnel, it and the frame. Some kind of stainless steel, with massive hinges. And very, very closed.

We walked up towards that door and all the time, in my head, I had this quote going around and around. You know the one, from the Lord of the Rings – 'The way is shut. It was made by those who are Dead. And the Dead keep it. The way is shut.' That bloody quote kept ringing through my head.

As we got closer to the end we saw something else though – there was another doorway, far smaller, in the right-hand wall as it approached the door. This was smaller, a dark cavity. It looked like a checkpoint, or a place for a guard to stand.

Not that we took any chances on it. I pulled out a disco ball and gestured to the others to take station as we approached it. The K unit wasn't picking up anything, but I still wanted to be careful about it.

Sorry – 'disco ball'?

Ah, sorry. It's a small ball, covered in LEDs. It activates about five seconds after you roll it and then gives you a great little light show. Zs go for the things like a moth to a lamp. Something about the pattern of lights or something. It's a great tool for fighting Zs in an enclosed environment. You can use it to draw them out if you suspect that they're there – or you can just distract them and then run like hell if there are too many of them.

Anyway, I rolled it over to the entrance, it did its thing – and nothing. Not a thing moved. We moved up and carefully checked it out – and it was empty. Just an alcove with a few nails in the wall, perhaps for clipboards or something. There were some faded patches on the wall, perhaps where something had been placed against it, but apart from that, nothing.

Well, nothing apart from a single phone jack about three feet off the ground. We didn't see that at first, but then the moment we did I called in Tomonaga, our electronics expert. She looked at it, griped that it had to be 40 years old and that it could probably detect morse code and then she plugged in her sniffer – that's an electronic device that detects electrical currents and any form of telecommunication links. She stared down at it for while and then announced that the phone line was dead as a proverbial doornail.

I told her to follow procedure and plug in an earwig – another one of the devices that we've developed. We get into all kinds of areas when we sweep for Zs and when we're in urban zone then there's a good chance that we'll find offices or other areas with computers somewhere. Most of them are dead, but we use earwigs to see if there's anything still alive in the system. They just listen to telecommunications lines and transmit if they find anything in the way of activity or modems. Of course in a tunnel like that then any transmissions would be hard to pick up, so we stuck a relay unit halfway along the tunnel and another at the entrance.

While that was going I was looking at that door. Close up it looked… well, massive. Old too. There was no corrosion on it, but it just looked like something out of a 1960's Bond film. And it was cold. I put my hand on it and I remember thinking that it was as cold as the grave, and then I pulled my hand back quickly.

As we all walked back to the entrance I took a look at my watch and saw that it was just past 2.30pm. I guess that all the tension had suppressed my appetite, but all of a sudden my stomach growled and I told the troops to take turns to eat. We didn't have anything fancy in the way of rations, just power bars and bottled water, but it did the trick.

The problem was that we were running out of time. The most important thing in any mission like that is that you have to be back at base by the time the light fades. Darkness is your biggest enemy.

So, when we factored in the time, when sunset was and the amount of time the journey back would take, we had just enough daylight left to check the second tunnel only.

We did that one the same way as the first. Majewski went up to check out the camera just in case. That one was in a worse condition than the first tunnel. Then we swept it. Again the floor was wet and the ceiling was damp and again there was a huge closed door at the end, along with an empty alcove and a phone socket. We installed another earwig and relays and then we got back aboard the Rhinos and we head back down to the base.

It was an easy trip – one or two Zs along the way, but that was it really. We got back to discover that it had been a quiet day there – they'd sent out a few patrols to get soil samples for the eggheads to analyse, but they hadn't run into anything much, apart from 2 Zs. We gave them our own soil samples, which we'd picked up at the tunnel site and at the points when we'd stopped, and that was us effectively done for the day. We drew rations, I sent a report to Seoul, telling them what we'd found and that we were going back to check out the remaining tunnels the next day and then I fell into my cot and I went out like a light.

That night we had another storm – and this one was a big bastard, it went right almost overhead of us. That's never a good thing, because the last thing you want is for something to be hit by lightning. The other thing is that Zs have a nasty habit of trying to follow storms – all that light and noise attracts them somehow.

We stood to at midnight just to be on the safe side, and it was a good thing we did. At about 1am we had a pack of about 150 Zs come shambling out of the night, howling at the storm. Didn't take long to take care of them, along with the occasional straggler afterwards. We stood down about an hour and a half later, with the storm gone and no more Zs coming in.

(His face tightens slightly and he looks out at the hills to our north.)

The next day we loaded up again and headed back. It was an easy trip, although we did have to stop once to clear a tree from the road that had fallen overnight, probably because of the rain. We didn't even see any Zs, which was nice.

Once we got back we did sweep the area again, searching the bunkers and the building, the trucks and the first two tunnels, just in case any wandering Zs had dropped in during the night, and then once we were clear we turned our attention to the third tunnel. Same procedure – Majewski checked the camera, it was declared non-functioning, we swept the tunnel and found a closed door and an empty alcove and then we set up an earwig. No sign of any life or any unlife. Same with the fourth tunnel.

The last tunnel though… that was the one that set off my sixth sense like a fucking fire alarm. The camera was visibly broken – it was swinging by its lead and at some point the wind had caught it and smashed it against the roof of the tunnel, so only part of it was still there – but there was something about that tunnel that made me feel very uneasy from the moment that I set foot in it.

I think we all felt it, because I saw a lot of tense body language all of a sudden. You don't get to go on these missions and not notice when something is wrong about your location, even if you can't work out what it is exactly. Mr B really felt it. His ears went back a little and his stance altered as if he was about to fight.

That feeling got worse as we went down the tunnel. This one was just like the other ones and we could soon see the shape of the closed metal door at the end, along with the alcove. Then, about 20 yards away from the alcove, everything changed, because it was then that Mr B's ears went flat and he growled. It wasn't the usual growl that said that there were Zs in the area, this was just a low warning rumble. Katherine looked at him and she looked at me. "Trouble" she whispered, and I nodded.

We couldn't see anything ahead – so it had to be the alcove. I pulled out the disco ball and then waited whilst everyone got ready – and then I rolled it down the tunnel. As it started doing its thing I tensed and raised my rifle… and nothing happened. No motion, no noise, nothing. We waited for a tense few minutes – the alcove could have been far deeper than the other ones – and then we carefully approached it and shone our torches in. Nothing. Just an empty alcove. We checked the ceiling just in case – hell we even tapped on the walls to make sure that there wasn't a secret entrance in there – but, well, nothing.

There was something though – a smell. It was faint, but it was there and it was… growing slowly. We just didn't know where it was coming from. Eventually I decided that enough was enough, told Tomonaga to install the earwig and the relays and then I went over to the door to collect the disco ball.

That door was colder than the others. Not much, but it was colder, I'm sure of it. And then, when I leant over to reach for the ball, I noticed two things. The first was that the surface of the ball was covered in this black slime, which made me pull my hand back in revulsion before I could touch it. Second thing was that as I bent over I could see where the smell was coming from. It was the black ooze by the door. There wasn't a lot of it, but as my nose got closer I could smell it quite horribly.

I called the others over and then I noticed it. The source of the filthy stuff. It was oozing, very slowly, out between the door and the frame. At first it was just in the middle of the door, but after a few minutes it started coming out of a few other places. And it all smelt of death. You didn't need more than a slight sniff to tell that. Something on the other side of that door was dead and rotting, and it was oozing through the door. Well, I say oozing, but by now it was dripping, which meant that there was a lot more water in it than before.

We had some collecting vials on us and we collected as much of the foulness as we could, before we finally dealt with the disco ball by carefully picking it up in the open mouth of a self-seal plastic bag that we usually used for large sample. Then we put that inside another bag, so that the smell would go away when we carried it, labeled everything and packed it away.

And then… We all just stood there for a long moment, staring at the bottom of the door and that growing pool of black… water I guess you could call it. I think we could all tell that something terrible had happened on the other side of that door, that whatever had happened to the North Koreans who entered that tunnel it couldn't have ended well. And then turned and walked back up that tunnel, walking away from the smell.

When we got to the top I told the others what we'd found and I had the other tunnel doors checked. They were all fine – so it might just have been a slight imperfection in the door, or the frame, that was allowing that stuff to seep through.

But that water… it told us so much. (He pauses and shakes his head, with an appalled look on his face.)

The fact that there was water at all was a very bad thing. My grandfather was a miner in South Wales, and the thing about any mine, or tunnel, or anything underground is that your number one enemy is water. You can have the best tunneling system in the world, but you'll always find that it's vulnerable to water. Underground springs, the water table, hell even just water seeping down through the rocks above after it rains. The first two things you install in a tunnel is a ventilation system and a pump, the first so that you can breathe and the second so that you don't drown.

If the tunnels were starting to fill up with water then that was a good sign that there was no-one left alive, in that area at least. It meant that either there was no power to the pumps, or that the pumps themselves weren't working. Under both scenarios that in turn meant that no-one was maintaining things.

When we got back to the base I explained what we'd found and we handed the samples over for analysis. I guess that we all had our suspicions about what those samples contained. Well, our worst fears were soon confirmed. The water contained human remains. Rotted human flesh and even some bone fragments. And those remains… well, a lot of them were saturated with Solanum. On the other side of that door was a lake of water that contained bodies – some Zs, maybe some not.

So much for the great Communist worker's paradise Kim Jung-Il must have told his people about. It looks like they shut themselves in the world's largest prison – and with the virus within the veins of at least one person. I can't think of anything more horrific – they locked themselves away from the virus, but they made it easier for the virus to do its work. There's irony for you.

So you think that they're all dead then?

Maybe. That part of the tunnel system at least is a dead end as far as life is concerned. Perhaps not when it comes to unlife, or at least the undead but… well, think about it. They all went down there and none of them have ever been heard from again. No signals, no sign of life… nothing. We've been going over all the pictures from orbit again, looking for something, anything… some sign of people existing in some form. If they were still down there then they'd have to pump out the tunnels – right, so there'd be some signs of them somewhere, a discoloration of the water coming out of the mountains or the rivers… heat sources somewhere from power sources, ventilation from waste gasses like CO2… But, well, we've seen nothing. Ok, so maybe they were paranoid enough to hide everything, to vent the gasses in a way that we can't detect… but we've been looking very hard in ways that they couldn't have predicted that we'd have by now.

And what the hell could they be eating by now? (He closes his eyes and winces) I take that back. I don't want to know.

We have to act on the assumption that all of them – or at least a substantial percentage of them – are Zs. The pre-war population of North Korea was 25 million people. As we have no idea what happened when they all went underground that means that we can't even guess at the number of potential Zs. Ten million? 14? Did they try to kill the infected? How many?

(Hughes leans forwards and then jabs a finger in the air.) And that's the problem. We don't know. There are potentially million of Zs under the ground over there in a tunnel system that we still haven't mapped properly. We had maps of the London Underground and the New York subway system, before we swept them. We've got nothing on North Korea. And judging by the condition of the tunnels that we saw, well let's just say that I'm not exactly filled with confidence with the durability of the area that the North Korean people are contained in.

None of the options we're facing are very good ones. Some people in Seoul want to just leave them down there to rot. I can see their point. Going in and clearing the tunnels would be a total fucking nightmare. It would be like Paris, only a million times worse. We'd have to map our way as we went, there'd be that stinking water everywhere, it's pitch black and I don't want to even think about how cold it is, and I really don't want to think about the number of Zs who are bobbing about in the damn place. Let's just say that casualties would be significant.

But the problem is that we don't know how long that tunnel system is going to stay self-contained. Communist construction has a tendency to be, well, shoddy. We've already noticed that some of Kim Jong-Il's more grandiose architectural monstrosities are showing signs of wear and tear. The west face of the Ryugyong Hotel has subsided, so Christ only when that thing's going to fall over.

My point is that we don't know if a doorway is going to fail at some point and the inhabitants of North Korea suddenly start seeing the light of day again. If the tunnel system is flooding with water every time it rains then the water pressure must be building up in all kinds of places. We just… don't know.

There has to be some way of mapping the system. What about ground-penetrating radar?

That's been mentioned. Problem is that it tends to be quite short range. You'd need teams of people equipped with the latest devices trundling around all over the country to even get a basic idea of the system. Maybe there's something in Pyongyang, some handy computer that will tell us everything in one of their ministries, but I wouldn't bet money on that. If they stripped a guardhouse of bunks then I don't think that they'd have left anything around with that kind of information laying around for anyone from the filthy decadent outside world to find.

No, we have exactly one information point, and that's a slim one. As we were packing up the base we got a faint quiver on one of our earwigs. It was picking up a computer with a modem under there somewhere. A dying computer come to that, one operating on reserve backup power. It just seemed to be trying to deliver a report of some kind, but we've never been able to pick up anything else at all on where it was delivering to.

We warned Seoul about it and they've been using it for the past week or two, trying to get access to the computer. They've been able to download some data, but it's all still being analysed.

(He shrugs.) Maybe it's just a supply list. Maybe it says how many rolls of toilet paper they took down with them. Maybe it has information about what happened. We don't know yet.

(He leans forwards again.) I do know one thing though. I'm never going back there. That country smells of death. No, Katherine and I are getting married and then we're going to start screwing like bunnies. We both want kids – we've seen far too many dead ones. North Korea's a problem for someone else. But I tell you one thing for sure – it's not going to be pleasant.