Author's Notes: Yep, it's me, crossposting more shit from AO3. This time, though, it's a fic I've only just started. Because of that, I'll probably end up updating this simultaneously in both places, meaning that for once the lovely people who read my stuff on FFN won't be way behind.
Also, just as a heads up, there are some minor (and not so minor) changes I've made to the world and canon plot of MHA. Firstly, Mutation quirks are way rarer here than in canon, and they're way more feared as a result. Basically, half the reason for the Underground existing is them being forced down there and out of actual society. Don't worry, though, some of our favorite mutant characters will creep back in...

Secondly, Izuku's backstory has been slightly altered, and Mina's has obviously been really altered; we'll get into both of those more as the fic progresses.

As usual, characters are aged up here; Mina and Izuku and the rest of Class 1-A are all 24.

This fic marks my first real stab at coherent worldbuilding, and I'll be doing it more throughout the story, so I'd love to hear your thoughts on it!

If you're interested, here's a link to a Discord server where I do tons of writing and brainstorming for all sorts of stuff, among a whole bunch of other shenanigans: /EdqrbRC

Enjoy!

Musutafu Underground: The colloquial name for the vast network of man-made tunnels, galleries, shops, housing, and public spaces underneath the central Japanese city of Musutafu, which is sometimes referred to as the Buried City. The Underground has its roots in the aftermath of the battle between the hero Craton and the villain Faultline, which has attained near-mythic status in the century since the early quirked legends destroyed each other under Musutafu. (Indeed, local folklore claims that the two have survived, and that the periodic localized earthquakes afflicting the Underground are a product of their continuing battle.) Following the devastating conflict, the city of Musutafu and the Japanese government worked together to develop the resulting tunnels into a new district of the city. Boasting more than a dozen distinct levels and stretching hundreds of feet down into bedrock reinforced by Craton's quirk, the uppermost regions of the Underground are world-famous for their open markets and underground gardens, and are a popular tourist destination. Further down, however, the Underground becomes an unnavigable tangle of illegal tunnels and unstable neighborhoods carved by the residents, who are largely the underclass of Japan, including a significant portion of the country's mutant population. The lower levels of the Underground are notorious for their infestation by criminal gangs and dangerous villains, the lack of any overarching plan or map of their incredible extent, and general absence of basic utilities. In fact, some experts have voiced concerns that frequent hero and police agency skirmishes with residents have stoked tensions with disadvantaged groups forced there by a hostile aboveground society, and that such tensions only increase crime rates and the danger of large-scale villain attacks…

-Wikipedia article "Musutafu Underground." Retrieved 10/06/2225.

It was a busy day in the Underground Markets. Tourists thronged around stalls selling all manner of souvenirs and delicacies, awed by the sheer novelty of their location. It wasn't every day you shopped in a market close to a hundred feet below bedrock, after all.

The uppermost layer of the Musutafu Underground was famous for its beauty; the whole cavern seemed to glow an eerie blue and green, thanks to the enormous lights that hung from the far-off ceiling, and the smaller lighting panels that ran along the ground and covered the flanks of the enormous pillars scattered across the Underground, supporting it. Between these monoliths, buildings rose like stalagmites, misshapen, carved from the rock itself; even so, they were neither ugly nor spartan. No, these buildings had an odd magnificence to them, a feeling aided by the way they blended seamlessly into each other, surrounded by enormous open areas with trees of all things. The Underground was full of gardens and parks, flourishing green spaces conceived, it sometimes seemed, out of nothing more than amusement at the natural order of the world. When men had quirks, what forced them to bow to nature?

When a single person-two, admittedly, though one had been a mutant, and therefore scarcely counted-could create such a magnificent place, what say did natural forces have in their destiny?

In the middle of the vast, cavernous first level, in pride of place, was the Market. It covered dozens of acres, sprawling out along straight-lined roads and weaving alleys alike. The filtered, pumped-in air rang with the cries of vendors and the coos of shoppers, the hum of business. And watching over it all, like sentinels against the night, rose the dueling statues at the heart of the Underground; Faultline and Craton, their battle immortalized in gargantuan hundred-foot masterworks of bronze, towering over every building in the Underground, visible from anywhere within the district which housed hundreds of thousands. It was a fitting tribute to the two legends, who had breathed their last somewhere deep in the earth under this spot, tearing each other to pieces even as they died-or so the legends said.

All in all, the Musutafu Underground, the product of a century of building and rebuilding, of men and women with quirks and iron hearts straining against the stubborn bedrock, appeared to be a testament to the glory of quirked humanity, proof of what could be accomplished by a society led and defended by heroes. And it was-so long as you didn't look too close, or delve too deep.

And, of course, any market attracted thieves like flies to rotten meat.

An unfortunate baker in the Market encountered one such lowlife as he turned to tend to his oven. As he watched, a figure in a dark cloak and full-coverage cloth mask surreptitiously swiped several loaves of fresh bread from where they had been lying for display in his stall. The figure began to walk away casually as soon as the bread had vanished under its cloak, doubtlessly vanishing into some hidden pack or pocket, utterly nonchalant.

The baker, naturally, objected, and raised his finger to point at the cloaked criminal. Raising the time-honored cry of indignant merchants, he bellowed, "STOP, THIEF!"

The thief, equally naturally, did not stop. Instead, they broke into a run, racing down a street packed with shoppers, who did nothing but stop and stare. They made it nearly the length of the street before an answering yell came from near the baker's stall.

"Never fear, citizen! We're here to help!" declared a caped hero who looked every inch the part of the fearless defender of the innocent, complete with brightly colored jumpsuit, elaborate hairstyle, and sparkling grin. He took a moment to pose, basking in the camera flashes from the tourists who clustered around, eager to take pictures of a real live hero in action.

Besides the showboater, two other heroes in leaner, darker-colored costumes rolled their eyes in perfect sync, then shot forwards like cross-country sprinters. One of them seemed to leave afterimages in her wake, while long metal spikes sprang eagerly from the hands and feet of the other. After a few moments, their colleague realized his glory was getting away, and took off in hot pursuit, trailing well behind his more alert allies.

The thief moved rapidly, but the heroes had years of training and expertise, and it wasn't long before they were closing in on the offender. A glowing hologram of the female hero suddenly surged forwards from her sprinting form, clawing at the trailing edge of the thief's cape as they turned a corner. The thief stumbled, an audible yelp coming from behind their mask. The heroes poured on the speed, knowing the chase was nearly over.

But then, the thief took a left into a dead-end alley, narrow and jagged from where two buildings had seemingly split apart. The heroes moved to cut off all escape, but without hesitating, the thief sprinted at the wall at the end of the alley, and leaped upwards. The heroes stared in sudden shock as the thief grabbed onto the perfectly smooth wall, which actually tilted so that the upper part was a large overhang. But the thief climbed upwards with seemingly little difficulty, finding handholds where there were none.

None of the heroes noticed the slight hissing from the thief's gloved hands, or from where their hands and feet had seemingly pressed into the rock as if it was melting, but they continued the chase undaunted. The showboater leaped into the air with powerful legs, hopping onto the slanted, uneven roofs to follow the thief with enormous bounds, while another flexed his hands to produce large, crampon-like spikes, which he began to use to climb much like the thief had. The third hero, lacking mobility, continued the chase on the ground, even though she knew there was no way she could contribute.

The thief turned their head, panting as they pushed their body further, desperate to get away. When the heroes appeared in their view, the thief turned back and muttered a soft curse as they began to gain again.

Soon, the thief was running out of fused rooftops to run along. Up ahead, a massive spur of rock loomed, perpendicular to the street they had been running along. Holes and windows glinted within it, revealing that the rock was, in fact, a colossal office building. The thief continued sprinting, not daring to slow down as the heroes continued to get closer and closer. At the last moment, the thief angled to the right, where a small lump rose out of the roof, shaped almost like a swirl or a skating half pipe. The thief ran up it, using it to redirect their momentum so that they could leap across a busy road below onto the side of the office building. Without missing a beat or even slowing down, the thief began running along the side of the building.

The spike-wielding hero, reaching the edge of the rooftop mere heartbeats after his quarry, could only stare in awe as the thief, one hand trailing along the rock as though finding purchase, moved swiftly and fluidly along the hundred-foot-long flank of the building. The edges of their feet seemed to press into the rock like putty, leaving the hero hopelessly confused about just what their opponent's quirk was.

(Of course, he was much too far away to notice the thin coating of incredibly potent acid that streamed through the tough, porous fabric of the thief's shoes, melting the rock into perfect footholds upon contact.)

The spike hero was out of the chase, and the hologram hero was nowhere to be found, but the chase was far from over. The showboater, it turned out, was actually half-competent; he continued leaping from rooftop to rooftop, easily keeping pace with the thief, searching for an opening.

He found it as the thief came towards the end of the megalithic office building, and as his rooftops similarly ended at one of the massive pillars that supported the whole Underground; as the thief looked frantically for the next avenue of escape, the hero made his move, crossing the distance between his rooftop and the side of the building in a fraction of a second. He smashed into the side of the building just a dozen feet or so ahead of the thief, his sheer strength shattering the rock around him, one hand anchored deep into the wall, the other outstretched to grab the thief, his grin eager and expectant.

The thief couldn't stop running; their position didn't allow them to hold securely to the wall if they stopped. But if they kept running, they would surely be captured. Seconds seemed to stretch into years as, under the mask, the thief's eyes scanned desperately for something, anything, to save them.

When they found it, those same eyes hardened in grim determination, even as their heart beat rapidly in anticipation of something even dumber and riskier than usual.

The distance closed; the hero reached out, grip ironclad, like an enormous roadblock that was impassable and immovable.

The thief got to within inches of that clutching hand, and closed their eyes. Then, they leaned in towards the building, and pushed off with the hand and foot still gripping the rock.

The hero's eyes went wide as his quarry flipped into the air, the top of their pinned-down hood flapping inches from his golden hair as the thief's cloak fluttered in the breeze of their movement, revealing the tight black bodysuit underneath.

For a timeless fraction of a second, the thief seemed to hang there, suspended in the air, legs tucked under them as they flipped forwards. Then, gravity reasserted its hold, and the thief plummeted downwards, towards seemingly certain death.

But their leap had pushed them forwards as well as upwards, and as the thief fell, they fell diagonally, towards their salvation: the structural support pillar on the opposite side of the road.

From a distance, the countless pillars which held up the roof of the Underground looked as small and insignificant as toothpicks, but up close, they were colossal; oblong "I" shapes twenty feet or more in diameter, hundreds of feet tall, made of dark gray concrete and metal alloys so dense and tough that a punch from the Number One Hero could scarcely dent it. Buildings all around it were seemingly fused to or built around the unbreakable pillar; who could blame them? It was by far the strongest structure in the Underground. Anchoring a building to it was excellent insurance against damage or collapse.

And today, a thief leaped desperately for that pillar, falling dozens of feet from the building opposite it as they got closer and closer to the ground. But then, with less than fifty feet to go before the thief became a red splatter on the hard stone of the Underground, they struck the side of the pillar. Instantly, the thief's acid-coated gloves were clawing at the concrete; even through the acid-resistant, extremely tough leather, the stinging sensation of losing skin on high-speed contact with the rough, uneven surface made the thief grit their teeth in pain. Nevertheless, they clenched their hands and dug the steel-reinforced, acidic toes of their boots into the pillar as well, fighting to slow their descent. They left inch-deep claw and scratch marks in the concrete as they gradually came to a stop, about twenty feet above the busy intersection. Already, there were tourists down there, pointing and aiming cameras at the thief.

Idiots, all of them. The Underground wasn't a place you came willingly…if you had any brains, at least.

Ignoring the sound of photos being taken, knowing that their identity was perfectly secure, the thief bit back pained grunts and hisses as they climbed down the pillar. Behind them, the hero, lacking the skill and strength to climb down, could only shake a fist dramatically at his foe's retreating back, before the crumbling rock around them drove him to clutch the wall even tighter to avoid falling.

Some time later, one of the enormous lifts that was the only way from the uppermost level of the Underground (the only way that appeared on maps, anyway) found itself with an unexpected and unknown passenger. As the group of people within slowly drained away with each stop at a subsequent level, none noticed the cloaked figure riding down on top of the colossal elevator, nursing raw-scraped palms and fussing over damaged gloves.

At last, though, the lift reached its final stop, and the thief slipped down from their perch, mingling in the small group of roughly-clothed men and women who quickly dispersed. Even this level, far dingier and smaller than the grand caverns of the first level, was no place for one of the thief's kind. There were fewer heroes here-fewer people to protect, and far less valuable property to defend-but there were still some. Instead of lingering, the thief snuck quietly through back alleys and forgettable tunnels, all slowly but inexorably tilting downwards, moving towards the bowels of the earth.

This leg of the journey was far longer, and by the time the thief could go no further down, there were practically no large lights like there had been in the higher, "official" levels of the Underground. No, the lights here, where there were any, were dim, red emergency lights scavenged from other tunnels, or patchwork homemade systems that only illuminated tiny islands in the sea of dark. No grand pillars held the roof up; the tunnels and caves were much, much smaller, barely big enough for four or five people to walk abreast in places, with homes hewn into caves or made of scrap. Even with the much smaller demand for support, these levels housed twice as many people as the upper ones, and collapses were all too common. Death by cave-in was an old friend to most residents.

This was the side of the Musutafu Underground that tourists never knew existed; indeed, the official position of the Japanese government was that it didn't exist, that the Underground had only eight levels, rather than twelve. Unofficially, though, this place, this warren of inhabited tunnels that stretched farther than anyone knew, was at the top of every government list of the biggest internal threats to national security, a breeding ground for every kind of monster under the sun, from organized crime to domestic terrorists. It was a place of the discarded, those driven from the surface and even the upper levels of the underground, those who were not welcome anywhere else.

It was the Depths, and to the thief, it was home sweet home.

They moved swiftly; there were no heroes here, but that didn't mean they were safe. As always in such a place, gangs of the desperate preyed on the vulnerable and unwary. The only solution, of course, was to be neither.

As they strode through the deepest pits of humanity in a cloak that hid every inch of skin, the thief was lucky in this regard; there were no laws in the Depths, but the closest thing to one might have been "Leave those who want to be left alone the fuck alone." It was a good law, for many reasons, ranging from not getting murdered and having your corpse sucked of all its blood, to not learning just what kind of person-or worse-ended up here. There were definite benefits to life, limb, and peace of mind by minding your own damn business in the Depths, and that suited the thief just fine. After all, they very much wanted to be left alone.

At last, the thief came to a sheer rock wall near one edge of the Depths; at the very top, near the point where the ceiling of the larger gallery met the wall, there was a small opening, barely noticeable from the ground. Taking a running start despite groaning muscles, the thief clambered along the small melted handholds in the rock, years of practice ensuring that they never even slipped. Soon, the thief disappeared inside the tiny hole, utterly invisible to the outside world.

After a few seconds of wriggling, the thief could stand up again, inside a tiny, cluttered hollow melted right from the rock itself. The floor was just big enough to contain a pile of rumpled, patchy blankets and pillows, not to mention considerable amounts of junk and discarded trash. Hidden among the debris were valuables-a stash of canned food here, a hoard of water jugs there, and other such items needed for survival.

With a sigh, the thief let the bag with the now-cooled bread drop to the ground. They pulled back their hood, revealing two curved yellow horns.

Soon, the rest of the cloak and the mask followed, revealing skin as pink as cherry blossoms, dark sclera the color of night, and golden eyes that could sparkle with light or flow like liquid metal depending on the mood.

With a sigh, Mina Ashido shook out her tangled pink hair, which was shaved down on the back and sides in an undercut, leaving the wild curls on top to flow down over the dark skin that had been exposed, winding around the backs of her ears. The old, healed scar under one eye, an angry slash in dark red, much darker than an ordinary person would have had, only enhanced the rebellious, dangerous air that clung to her. Maybe it was something in her eyes that did it; the look there was eternally wary, ready to spring into deadly action at the drop of a hat, yet also warm and entrancing. It was the look of a predator and the look of prey all at once.

Stretching with a groan, Mina grabbed one of the loaves and flopped down into the nest-like pile of blankets that served as her bed.

"Jeez," she muttered to herself as she rubbed her eyes in exhaustion, "what a fucking day, huh?"

Mina took a bite from one end of the loaf, savoring the taste; she didn't get stuff like this that often, since stealing in the upper levels was so dangerous and risky. But damn was it worth it, if only for the special occasion.

Mina wolfed the first loaf down in seconds, then decided to try and savor the second one. As she took smaller, dainty bites, she said softly, "Happy birthday to me, I guess."

Time was hard down here, Mina knew, but she did know today's date thanks to the clocks in the upper levels. She'd seen them while looking for some food to steal.

July thirtieth. Her twenty-fourth birthday. For all that that meant down here.

"Man," Mina decided, "time sure fucking flies, huh?"

Despite her best efforts, the second loaf was soon gone, too. Not wanting to waste all her hard-earned food, Mina decided that that was enough, and set the other two loaves aside for tomorrow. She was thankful her stomach didn't rumble much; it was as used as she was to food being as reliable as her ability to steal it.

Feeling the pull of exhaustion, Mina yawned deeply, then figured that, even if she was gonna wake up sore as hell tomorrow, she might as well get some rest before then. Sleepily, she muttered, "Just another day in the Underground."

Then, she was asleep, snuggling into the pile of blankets. Outside, life went on, in its messy, desperate way. For now, though, the pink-skinned woman, not welcome in the world above, slumbered peacefully in the depths of the earth.

It couldn't last forever.