He swam to the far wall, making sure Sam was with him. "In there. Now!"
Sam looked at him like he was crazy. "We'll be smashed."
"It's the only way out." Dean looked up. Another crack was forming just above them, jarring another slab loose. "Go, now!" Without waiting, Dean shoved Sam's head under the water, pushing him into the dark hole and following behind just as the slab crashed into the water and the resulting wave rocked over them. The headlamp scraped off the tunnel's edge, plunging them in darkness.
Dean moved forward, bracing his hands along the jaggedly curved walls until he ran into Sam. His brother wasn't moving, probably waiting for him. Dean shoved against him, knowing the tunnel was long and the tide hadn't gone out yet to provide any pockets of air near the top.
Sam slapped out at Dean, still not moving. Dammit, Dean didn't know what Sam he was dealing with. The coherent one or the out of it Sam? The kid could be disoriented again, not cognizant enough to know what was going on. Quit fighting me! Dean pulled himself up along Sam, his back scraping the top of the tunnel. Sam's arms flailed at him, legs kicking and thumping against his. Could he not tell the difference between legs and a Nereid's tail? The water rushed against them. He couldn't see, but Dean felt the tunnel shifting. If it split open on them now, they really would be crushed.
They had to go. Dean grabbed at Sam, but he couldn't get a hold on the smooth skin or flapping gangly limbs. Except for the waistband of his boxers that were out of reach there was nothing to hang onto. The hell there wasn't. Dean reached between the slapping hands, tangling his fingers into strands of overlong hair and pulled. Long hands clamped around his wrist instantly, tugging to get free, but Dean wasn't about to let go.
If he let go, he'd lose Sam and that wasn't an option.
Kicking off of the walls because Sam's thrashing legs were in the way, Dean practically walked them through the tunnel, bare feet getting torn on the uneven walls. His brother was in full out fight or flight mode and the kid was fighting for all his worth, alternately sending jabs into Dean's ribs and trying to yank his hand out of his hair, but no way in hell was Dean letting go.
Fighting Sam was taking more of his breath than Dean had. Come to think of it, he had slammed Sam into the tunnel before his brother could take a proper breath and with how badly Sam was thrashing . . . Not good, not good. Dean redoubled his efforts, pulling Sam with everything he had left. He finally managed to hook an arm beneath Sam's armpit and heaved. In the darkness, he had no idea how much farther they had to go or even if the opening on the other end was still there or already fallen in. He jolted briefly, wondering if he was even going in the right direction.
Had to be. His struggle with Sam couldn't have gotten them turned around. Could it? Shit. He couldn't think like that. Keep going, just keep going. He pushed on, suddenly realizing that Sam was no longer struggling. Wasn't swimming on his own either. Dammit. Made the going easier, but . . . dammit.
The tunnel narrowed and Dean had to pull himself on ahead, extending his and Sam's arms out to guide Sam through after him. He felt a rock fall between them and he lost his grip. Felt Sam float away.
The tunnel was collapsing and he'd lost Sam.
Not happening. There wasn't enough room to turn, so Dean used the fallen rock to push himself backwards, squeezing between it and the walls. His feet connected with something silky. Had to be Sam's hair. Scooting back more, Dean reached, found Sam floating against the ceiling. Grabbing Sam's arm, Dean pulled himself past the rock, maneuvered Sam past it, feeling the tug as gigantor must have scraped against it.
Dean winced on his behalf but kept pulling. That's what his brother got for making him do all the work when they really couldn't afford it.
He pulled Sam through the darkness, chest about ready to burst when bubbles raced across his flesh, followed by a rolling current that pushed them forward like a sudden water slide as the cavern collapsed behind them. The wave spewed them out of the tunnel, ripping Sam away and Dean found himself clawing to the surface and the morning air, gasping in huge breaths as the cliff wall behind him rumbled, falling several feet over the hole they'd just exited, forever altering the dipping shape of the landscape above.
Closer, barely registering over the roar of the shifting, grinding stone, wet choking gasps rolled over him. "Sam!" He swam over to his sibling who was gasping, watching the high wall sinking several yards with a horrified expression.
When Sam's head slipped beneath the water, Dean hauled him back up, and kept him afloat until his sputtering stopped, so glad his brother was conscious again, though it would have been nice if he'd come to earlier.
"Did . . ." Sam managed to get out between long inhalations. ". . . we . . . do that?"
Dean's mouth pulled down in his own facial shrug. "Well, you're the one who threw the grenade."
If possible, Sam's features grew even more horrified.
Hiding his smile, Dean circled around to locate their boat and did a double-take when he saw a fisherman and a streamlined skiff not four yards away. Slack-jawed the guy looked from the large slabs falling from the sagging cliff, creating spectacular splashes, to the brothers, a fishing rod about to fall from his loose fingers.
Dean lifted a dripping arm and waved. "Hey, a little help here!"
He wasn't sure what he was going to come up with to explain the collapsing cavern or why they were out here in little more than their underwear, but that was okay. The way Dean saw it, they had killed all the mermaids, destroyed the eggs, and he'd gotten Sam out. Sure Sam was still a little out of it and he had a nasty bite and claw scratches they'd have to take care of, he'd have to be watched for secondary drowning with all that salt water in his lungs, possibly go to the hospital to make sure, but Sam was alive. The kid was alive.
All in all, Dean'd chalk it up to a successful hunt.