a/n: welcome, friends, to my first attempt at a multi-chapter fic in ages. this prologue is meant to serve as a teaser to what will ideally come in forward-completed chapters later on. as one of my more ambitious fics in both content and length, i'm nervous to put this one out there, but i hope that you enjoy the ride!

lastly, thanks to my good friends and longtime fandom buddies ash and pi for soundboarding as well as beta reading. without them, i would have rejected this fic as too outlandish to ever see the light of day.

-angel

Leonardo peers down the tunnel's mouth.

"Something isn't right."

"No shit, Fearless," Raph says from his perch atop a hill of rubble. He's just as tense as Leo is; maybe more. "If it was, we wouldn't be out on this little field trip."

"That's not what I mean."

Leo doesn't expand further than that, nor do his brothers press him, but the whole atmosphere seems to tighten. Michelangelo swerves to his immediate older brother, resting his hands on his nunchucks.

"You getting anything, Don?"

"Negative," Donatello replies from the ground. He hits a few keys on the display of his portable seismograph, then sighs and closes the lid. Snapping the clasps shut, he hauls it up by the handle attached to its side. "Let's keep moving."

They follow, Leo and Mikey detaching from the walls and Raph sliding down from his pile of debris. Mikey picks up his pace to keep even with Don.

"I'm kinda with Leo on this one," he says nervously. "This place gives me the heebie-jeebies. Besides, we've been out here for hours! Can't we call it a day?"

"I don't recall mentioning any 'heebie-jeebies'," Leo comments dryly from the rear.

Raph throws up his arms. "What's gotten into you two? It's just a freakin' tunnel!"

"It is not just a tunnel." Don's tone is clipped as he hurries along, carefully sidestepping a fissure in the ground. "I told you guys-there's been some seriously weird activity going on down here the past few weeks. It's getting closer and closer to the lair, and as far as I can tell, there's nothing stopping it from reaching us."

Michelangelo rolls his eyes. "Should I be more weirded out that one day you just thought, 'hey, let's monitor the planet for earthquakes near the lair!', or that you actually found some and decided we should all go towards them?"

"Installing underground sensors in the area was my idea," Leo admits, still squinting at the walls like they're about to attack. "But I was thinking more along the lines of motion sensors."

Don shrugs. "I was bored."

Raph grunts, following Leo's gaze. "Now that you mention it… these tunnels don't look natural. You think someone's been diggin' the area up?"

"Would that even register on Donnie's size-o-meter?" asks Mikey.

"Seismometer." Donatello frowns, rubbing his chin. "And… maybe. Calibrating the oscillators was tricky for such small models; I wanted a better output than -"

"Layman's terms, Don," Leo interrupts sharply.

"I've been measuring vibrations in the area, not necessarily seismic activity. Theoretically speaking, construction on a big enough scale could have tipped the sensors off."

"Big? How big are we talkin'?" Raph demands.

Don scratches the back of his neck. "Um… 'secret underground facility' big?"

"I think it's time to pull back and regroup," Leo says. "We don't know what we're dealing with yet."

"Aw come on, Leo, don't be such a-"

Before Raph can finish, a shrill alarm pierces through the air. They wince, Mikey covering his ears, as Donatello curses and unhooks a small digital scanner from his kit.

"Geez, Donnie, give a turtle a warning-" He freezes, realizing that Don has just turned several shades paler. "Don?"

"We've got movement," he breathes, eyes wide. "And it's coming this way. Guys, we need to move." He looks up, to each of them, his expression turning fierce just as an ominous rumbling begins echoing through the tunnel walls. "Now!"

The earth suddenly lurches beneath their feet, hard enough to send them all off-balance, and Don's seismometer kit clatters to the floor as it's thrown from his grip. Raphael is the first to recover; he yanks at Mikey's arm and gives Leo a rough shove, piloting them back the way they came.

"You heard him! Let's get the hell outta here!"

Only years spent training as a single, well-oiled machine keeps them from tripping over one another as they careen backwards through the tunnel like madmen, trying to outrun the echoes of groaning rock behind them. They're wordless and breathless for what feels like hours; shoving at each other when one slows, grabbing one another's arms to counterbalance whenever the ground rolls underfoot. At a sharp bend, the ground gives an especially violent heave and Don's feet skid. He recovers, scrabbling at the earth and launching forward again at top speed, but Mike's already looking behind him to make sure he hasn't fallen behind. Whatever else he sees cuts a startled yell from him.

"Guys, faster!"

"Go, go!" Don cries, already aware of the problem.

The words are barely out of his mouth before the inner wall collapses in front of him, sliding into the bend in an avalanche of dirt and cracked stone. Mikey yelps again and dives for safety, disappearing in a curtain of raining stone. He can't even see Leo-

Donatello realizes in a rush that he won't make it. He lurches to a halt, heart plunging nauseatingly into his stomach. A roar sounds just behind him, and Don reaches out to snag a blur of red and green, stopping his brother from bludgeoning his way through an active tumble of rocks the size of his shell; Raph curses as his arm snaps against Don's grip.

"Raph, no!" Don's voice is eerily level despite its volume, and he wonders if his brother will be so quick to process their predicament. "Don't-"

"Let go of me! Don, we gotta get out!"

He opens his mouth to respond, but a cry of pain from one of his brothers up ahead draws his head sharply up like a beacon. As he roughly hauls Raph towards the stable wall, it dawns on him that they're all in real trouble - the debris is rapidly stacking to the ceiling. There's no way of getting to the others like this. They're as good as separated.

Donatello's brain is firing on all cylinders. A crack opens up between his toes, and he shifts rapidly out of the way, stomach leaping into his throat. Raph wrenches from his grasp with a roar, both sai flying from his belt to attack the wall of rubble with vigor.

Helplessly, Don looks behind them, to the rain of dust and crackling stone, to the ominous rumble still approaching from the darkness. A split in the ground is crawling towards them at a furious pace, swallowing shadows like a ravenous beast.

He doesn't know what to do. All of his improvisations in the past - those moments of brilliant insight that have pulled him and his brothers from the jaws of death, and he's stuck here miles underground in the dark, powerless against one single act of nature.

So he does the only thing he can think of.

"Leo! Mikey! Keep going!"

"No!" Comes the immediate, muffled shrill from the other side, and that is definitely Mikey; Don doesn't give himself the time to wonder if that means Leo's the one who cried out earlier, or if their fearless leader is just too busy attacking the other side of the wall to comment.

Don backs closer to the landslide, carefully avoiding Raph's warpath. He feels sick. He doesn't want to die. He doesn't want Raph to die, he doesn't want Leo or Mikey to die, but cold reality is holding strong and he's just desperate for someone to get out of here alive. "Don't be stupid! You can still get to safety!"

"You don't be stupid!"

"He's right," Leo's voice interrupts, the acoustics all wobbled and blurred. "You go, Mikey! Get Casey, get Leatherhead, get -"

"No! I'm not leaving any of you! Raph, tell them!"

"Just hang on," Raph seethes. There's a meaty crack and a sharp intake of breath as he snatches a hand back from the stone. His sai clatters to the ground. Relentlessly, he hacks at the wall with the remaining one, his pace refusing to slow. "I can get us all outta here. I can -"

Something shifts again in the air; seems to swallow all of the oxygen, Leo's muffled reply, everything left. Raph freezes, his eyes lifting to the ceiling. The beam from his mounted flashlight illuminates a flurry of stale dust, swirling like stars; beyond that, a fissure angrily opening in the ceiling, staring down at them like a slitted mouth with jagged, stony teeth.

A sound of crackling bone overhead brings the world to a pinpoint. Another muffled noise of metal clanging sharply to the dirt.

"No," comes Leo's voice.

Then the mouth screams open, and the darkness plunges down.