Darien could feel his nerves beginning to jangle as the door to the Coring Well rolled open in front of them. Between whatever it was that was stalking them through the city depths and the prospect of what they might find within, he could feel the control he'd been exerting over his mind and emotions beginning to slip.

The sight that awaited them didn't help.

The Coring Well was vast, shaped like an enormous dome that could have housed the whole of Blink Headquarters with room to spare. Gargantuan ribs of solid stone rose up like the spokes of a wheel to meet in the centre of the ceiling a hundred feet overhead. A central sphere held the whole structure together: a solid black globe the size of a house.

Directly below it lay an immense circle of water ringed with dozens of ramps, each one wide enough to drive a cargo freighter down. The whole chamber seemed to be designed like a gigantic diving bell but Darien found his attention drawn away from the architecture, instead focusing on the tremendous debris field that surrounded the well itself. There were more bodies, dozens – maybe hundreds – scattered all around the dry-dock area, as well as the wreckage of several vessels that he assumed had once been submersibles.

As long as a colonial frigate, each of the massive subs was shaped like a triangular prism, with a flattened out nose-cone where long bands of opaque glass provided windows. While a couple had survived mostly intact, others lay broken in half or worse, spilling their innards and crew over the floor of the Coring Well. Their thick plates of brass-coloured metal hadn't been enough to withstand whatever force had torn through the place and flung their huge structures around like leaves in the wind.

"Looks like a war zone," Hekket commented quietly.

Darien shook his head. "More like a tomb."

"I guess their plan to get out didn't work," Idas said, instinctively raising the barrel of his new weapon to scan the area. "What do you wanna do?"

"What we came to do," he replied. "Come on." He led his companions on, carefully picking his way through the millennia old graveyard.

These corpses were not as well preserved as those they'd encountered before, and instead of decay the air had a salty, metallic tang to it. Darien placed his feet carefully, stepping over limbs and around debris, giving the bodies a wide berth. Something about them still made him uneasy.

Upon closer examination he could see that some of them sported the same circular puncture marks as the other unfortunate group. The cause of death for many of them, however, looked much simpler. Limbs were bent at unnatural angles and spurs of broken bone could be seen through the last vestiges of flesh and leathery skin. The force that had wrecked the transports had inflicted that same fury on their comparatively fragile bodies.

Skirting around the carcass of one of the ships, Darien glanced inside at its interior mechanisms. Like the city itself, the passages contained within it were constructed with high arching roofs, packed together in a honeycomb arrangement. More bodies lay within.

He looked away, gingerly stepping over a particularly mangled corpse and coming to the edge of the Well itself. His sharp eyes quickly noted the small cracks in the stone that emanated out from the water ring – small in comparison to what they'd seen before, but still large in their own right. They snaked out dozens of feet like oily tendrils. Even the hugely reinforced structure of this place hadn't completely withstood the seismic forces at work.

There were no lights in the water. All Darien could see when he shone his torch into it was a seemingly bottomless black hole. An uncomfortable feeling settled in the base of his stomach. Beside him Hekket leaned forward, holding his mapper in one hand out over the water. The machine pulsed and the other boy looked at the display.

"It's just like the main schematic showed," he said. "Something's blocked the well about a hundred and fifty metres deep."

"Isn't that cutting things close for the breathers?" Idas pointed out.

Darien shook his head. "Operational specs give a two-hundred metre max. We'll be fine." Then he broke off when he saw the surface of the well beginning to ripple. He motioned the others away from the edge as a low rumble reverberated through the dome. His companions scampered back from the water, but when the quake arrived it stuck with considerably lesser force than before. Darien couldn't tell if it was a gentler shock, or if the reinforced walls of the Coring Well simply withstood the forces more securely.

When they reconvened next to the water's edge again, however, Hekket delivered a surprising piece of news.

"Darien," he began uncertainly, staring at the display on his mapper. "Whatever's blocking the well...it's shifted."

Idas gave him a quizzical look. "Shifted?"

"Yeah, it moved with the quake. It's big enough – maybe whatever that thing is moving around against the superstructure is causing the tremors in the rest of the city?"

"So now what?"

Darien gave a dread-laden sigh, knowing full well what they needed to do. While they'd identified something sitting at the base of the well, they still didn't have the slightest clue as to what it was or how to deal with it.

"We go down," he said simply.

A moment of uncomfortable silence hung in the air after his resigned proclamation. They all knew what needed to be done, but looking into the space-dark pit of water Darien was hardly thrilled by the prospect. Instead of speaking he opened up his pack and freed a climbing tether, handing one end to Hekket.

"Idas, you stay here," Darien ordered.

Hekket pulled a face. "How come he gets to stay?"

"One – we don't know if that cannon can fire under water, and two – you're the better swimmer."

"Since when?"

"Just hook up the damn tether."

The medic gave a disgruntled snort, clipping the tether to his belt. Once they'd both attached their breathers and goggles, Darien turned to Idas.

"Stay sharp," he said. "I'm not sure we're alone down here. If you see anything at all, radio us. We'll be back topside as soon as you send up a flare."

"I'll be fine." The burly operative patted the barrel of his cannon affectionately. "Any uninvited guests are going to get a warm welcome."

"Look after yourself." Darien clapped him on the shoulder, then turned with Hekket to face the unwelcoming glassy surface of the Coring Well. The other boy looked thoroughly unhappy about the situation, his face creasing into a grimace as he stared into the water. The two operatives exchanged looks, but Darien shoved his fear to the bottom of his mind. He didn't need to say anything more – he simply stepped over the side and plunged into the water.

Side by side the two operatives sank into the depths.

It was pitch black inside the Coring Well, and rather than try to rely on the carbine-mounted torches to light the way, Darien instead activated one of the powerful glow nodes and clipped it to his belt, instructing Hekket to the same. Two spheres of white floated in the dark, drifting down into the unknown.

"Idas, can you read me?" he said quietly into the comm, as though speaking loudly might disturb whatever was lurking in the deep.

"Yeah, I hear you," Idas replied. "Local signal seems solid. How's it looking?"

"It's dark."

"Not going to lie – it's pretty creepy up here, hanging around with these...things, but other than that it's all quiet."

"Don't worry, we're not going to be down here for any longer than we have to."

A hundred metres deep Darien spotted a massive dark shape in the gloom and swung his torch quickly to illuminate it. Relief shuddered through him when he saw what looked simply like a thick pillar of rock. It lay at a strange diagonal angle through, bent into sections in a rough arch. Following the line of the pillar he saw that it was buried into the thick wall of the Coring Well, with cracks radiating out from it in all directions.

"Looks like a collapsed strut," Hekket commented. "What do you think?"

"As good a guess as any," Darien replied, letting his torch run over the sections of the pillar. The kinks in its structure looked deliberate – almost uniform in their length, but the way it had plunged into the opposite wall seemed anything but planned.

"We're fifty meters from...whatever it is. We should get a visual soon," his companion warned.

Darien made a conscious effort to keep his heart rate and his breathing under control as they delved deeper. What could be waiting for them? Had the structure of the Coring Well collapsed in on itself all those years ago? Had there been a tectonic shift sometime over the millennia that had driven a blockage into place? But if so, why had the tremors and quakes started now, just hours after the under-water city had been rediscovered by civilisation. That was too much of a coincidence and it made his stomach knot.

It took another long, slow minute for the two operatives to descend far enough that Darien caught sight of their target in the gloom. The light from the torch and the glow-nodes rebounded back off of a huge rocky surface and he recalibrated the micro-jets, slowing his descent.

"You see it?" he asked, giving the tether a small tug.

"Yeah. Looks like a cliff face."

Hekket sounded calm enough – he was grateful for that. Hell, he was grateful for any company in this forsaken watering hole. The closer they drew to the obstruction the more the medic's description rang true. The surface spread out before them, curving away slightly in both directions to vanish into the darkness beyond their little bubble of light. It looked like rock, with jutting irregularities and protrusions, some of them streaked with what appeared to be a kind of indigenous seaweed.

His feet touched the ground. It was as unyielding as the stone of the city's construction, like landing on a gigantic boulder. Gingerly he started walking, reversing the thrust of the micro-jets to keep him from floating away. He turned his torch on Hekket at the other end of the tether, and found his companion bent down on one knee, examining the surface.

"What do you think?" he asked.

"Looks like rock," Hekket said. "But...there's something, something not quite right. I'm going to take a sample."

"You're not a geologist," Darien reminded him.

"Call it a hunch."

"Well make it quick. We need to examine the edges. If this big rock is shifting position and causing the quakes we might be able to find some way to brace it – to hold it in place."

"This won't take long."

Freeing a siphon-module from his backpack, Hekket set to work. The buzz of the little tube's mechanism was muffled by the weight of the water. Thousands of microscopic drill-bits dug into the rocky surface, coring out a tiny sample. As he worked, Darien turned his attention to further examination. He moved as far as the tether would allow, scrutinizing the cracked and scarred rock face. Long dark scratches blemished some areas – others rose up in small, uneven mounds. Upon closer inspection he could see bits of debris that were embedded into it – debris that bore an unsettling resemblance to the wreckage in the chamber above.

Then he came across a crevice maybe two feet across and a dozen in length. Unlike the other surface marks, this one looked like it belonged there as part of the cliff, with smooth, uniform edges. As he leaned forward to examine it he felt a strange pull like a current, trying to drag him inside. He planted his feet and jerked away, levelling his carbine automatically. For a few seconds nothing happened, but then a huge expulsion of bubbling water burst from the crack, the resulting force nearly knocking him over. Darien's brow creased in confusion.

"Finished," Hekket's triumphant announcement tore his attention away.

Then a noise groaned through the water around them, a low, painful bass throb that made Darien clamp his hands over his ears in a futile effort to block it out. Hekket jerked the siphon-module away as though he'd been scalded, scanning the water around them wide-eyed. He looked at Darien.

"What the hell was that?"

And then the ground beneath them started to move...