The dark passage looked about as inviting as a swamp, devoid of any light and filled with the icy water of Marianas's oceans. Amber crouched down at its edge, shining her torch into the depths, revealing more stairs leading down to the subsidiary passage. The actual structure looked intact and the water level wasn't rising. It struck her as a rather tranquil little spot despite everything.

"According to the mapper that passage runs a hundred metres and links with another stairwell," Hekket said, studying the read-out on his instrument. "That should bring us up into the passage on the other side of the door."

"And what if there's another locked door at the far end?" Brannigan asked, her eyes staring into the flooded passage.

"Then I guess we'll have to double back and try to find another route."

"There's one way to find out," Darien said. "Who wants to go for a swim?"

Amber smiled, shedding her backpack and combat vest. "I'll go."

"I love a volunteer." He took the pack from her, grinning. "No heroics while you're down there alright? Just get to the far end and confirm if we can get through that way."

She dipped her fingers experimentally into the water. It was very cold and she jerked back sharply, wrinkling her nose. "I don't think I'm going to be down there any longer than I have to."

Darien held her pack open while she rummaged around to find her breather mask. The contraptions were a godsend for anyone doing any kind of under water activity. Fitting over the nose and mouth, they were not unlike hospital oxygen masks in appearance, but had some crucial differences. Its rim vac-sealed against the wearer's skin, creating a water-tight barrier, and fitted to both sides of it were tiny compressed oxygen tanks. Officially it would provide her with three hours of air, give or take half an hour depending on how much she had to exert herself during that time. The main thing that made it invaluable was that she'd still be able to communicate while she used it.

Attaching the breather to her face, Amber felt a slight tingling sensation as it formed its seal around her nose and mouth. She slipped a pair of goggles in under the mono-rig, feeling the same tingle around her eyes as they sealed into place. Then she reached down to her boots and flicked the activation switches for the micro-jets that would propel her through the water. She checked to ensure her carbine was securely strapped to her body and the bandoleer of lances was firmly loaded, then looked to Darien.

"All set?" he asked.

"All set." Amber turned and faced the water. Even through she was wearing the breather she still took a deep breath before diving head first into the depths.

She gasped at the searing cold that enveloped her despite the protection of her Blink issue dry-suit. The oceans of Marianas were nothing short of freezing. Nevertheless, she took a couple of steadying breaths and pointed her toes. The micro-jets came to life, giving a gentle shunt of propulsion to drive her through the water, leaving her hands free. The mono-rig automatically adjusted for the sudden gloom of the water, generating a thin ray of light to accompany the torch mounted on her carbine.

"Amber, can you hear me?" Darien's voice sounded in her ear.

"Loud and clear," she answered.

"How's the water?"

"A bit colder than I'd like, but you can't have everything." She smiled wryly to herself and turned her body, moving slowly and smoothly through the water. The liquid had been totally still until her arrival, and she could see it rippling up against the roof. The flooded section still spanned a hefty twenty feet in height, and her light revealed more rooms – some with open doors, others sealed shut. She glided past them, taking care to give a good view through her mono-rig for the experts back on the sub.

Her torch swept over the floor and she saw a series of strange, symmetrical scratch marks that had bitten into the surface. "Thirty meters in," she reported. "Some signs of structural damage. Nothing major. Doesn't look like it's from the quake."

"Keep your eyes peeled," Darien replied. "Who knows what else those dolts back on the sub forgot to mention."

"You're making me feel all better."

"Sorry. Just stay sharp, okay?"

"Copy that."

She moved on, sliding effortlessly onward under the gentle propulsion of the micro-jets. The deeper she penetrated into the corridor the darker things became as the light from behind her faded. Soon she had to rely entirely on the gun mounted torch and the spotlight from the mono-rig to guide her. Despite the open space of the corridor itself the darkness closed in around her like a physical thing and Amber found herself wishing she hadn't been so eager to volunteer.

Pushing the apprehension out of her mind she flicked her ankles, kicking the force of the micro-jets up a gear. She began slipping through the watery shroud much more quickly, now ignoring the rooms and engravings she passed. Now she just wanted to find what they needed and get back to the comfort and safety of her squad mates.

It didn't take much longer until her torch mercifully swept over the base of the far stairwell and she felt herself relax at the sight. And better still, at the top of it she could see the tell-tale ripple of the surface of the water. The section above her was not flooded. "Darien, I see the other stairwell."

"Can we get through?"

"I think so, I can see the next passage and it's clear, no flooding...standby." The light from the torch also revealed something else in the gloom. She saw a long, dark wound in the floor just before the stairs started their climb – a large crack maybe six feet across and three times that in length that split the stone open. She shuddered to think at the kind of force that would have been required to cause it. "I think I've found where the water's come from. There's a fissure in the base of the passage. Looks like more seismic damage."

Amber was about to move on and swim up the stairway when the tiniest flicker of movement caught her eyes, just as she pulled her torch away from the crack. Instantly she stopped, clicking the micro-jets down to their minimum level. They held her upright, treading water while she levelled her carbine at the black maw. She forced herself to take steady breaths. Had she imagined it?

The light didn't cut very far into the shadows, only revealing the metallic glint of the cracked stone interior. Angling the micro-jets ever so slightly, she let herself drift a little closer. The light struck something else within the darkness and she stopped again, staring hard. It looked like a smooth, black boulder, wedged in place, half-blocking the tear in the structure. She edged a few inches closer.

"Darien, there seems to be something-," she began, but her words were abruptly cut off when the boulder exploded into life.

Amber didn't get a great look at it. She simply saw what had looked like a solid rock unfurl like an armadillo, revealing an arthropod creature the size of a car with claws as big as her. It surged out of the darkness in a whirl of churning water and she let out an involuntary shriek of horror. Rocking back on her heels, she fired a shot from her carbine then spun around, slamming her boots into a pointed position and driving the micro-jets into top gear. She hugged her carbine close to her body, turning herself into a human-shaped torpedo and shot off back towards the safety of her comrades.

"Amber? Amber, are you okay?!"

At first she ignored Darien's panicked voice in her ear. She could hear the thrashing water behind her as it gave chase and terror took hold. She willed herself into a more streamlined shape, scything through the flood-water. At last she managed to blurt out a response.

"There's something down here and it's chasing me," she babbled. "I'm getting the hell out of here!"

She forced herself not to look back – knowing it would slow her down by breaking her aerodynamic shape. Instead, she fastened her eyes onto the fast approaching glimmer of light from the corridor where the others were waiting. Just a few more seconds. The stairs raced towards her at an alarming pace, but Amber didn't dare slow down. She didn't even try to unfold and climb them. She simply pointed herself at the surface and prayed.

Amber exploded from the water, floundering and fighting for breath, adrenaline pounding in her veins as she tore the breather mask from her face. A pair of arms were waiting. Hekket caught her as she burst from the depths and heaved her bodily backwards with a force that belayed his spindly frame. She clung on desperately as he dragged her several meters back from the edge of the water, past the other operatives who all had their weapons trained on the flooded passage. Then they tripped and went crashing to the ground in a jumble of limbs.

A short volley of gunfire echoed through the passageway and she looked back from her prone position to see an enormous dark shape receding back down into the darkness. Darien and the others stayed frozen in position, spread out with their weapons trained on where the thing had been just seconds before. Seconds ticked by but it seemed the creature had no wish to come out of the water. Amber let out a gasp of relief and slumped against Hekket, letting her head lean against his chest. His arms stayed clamped protectively around her and for a long moment she lay there, glad of his presence.

When Amber managed to gather her breath, he helped her to her feet and placed a steadying hand on her shoulder, looking her up and down for any sign of injury. His eye shone with concern and when he spoke his voice was as close to panic as she'd ever heard.

"Are you alright?"

"Yeah, I think so." Amber turned her eyes skyward and gulped in a steadying breath before she met his gaze. "Thanks for...you know-"

"Don't mention it."

"All teams, this is Hammerhead," Darien barked, instantly thinking of the wider implications. "Be advised, we have live, hostile wildlife active in this place, apparently confined to flooded sections. Use extreme caution if exploring an under-water section of the city." He rounded on her and pointed back down into the water. "Just what in the black hell was that?"

Amber shook her head. "I have no idea. It was like the ugliest damn lobster you've ever seen. I didn't get a good look, but it's big, and it's got claws. It came out of the crack in the floor."

"More wildlife our good lieutenant decided not to mention," Idas growled and she could see the fury clearly etched across his features. Before anyone could vent any further anger, however, their comms crackled with an incoming message. An instant later, Link's voice sounded in their earpieces.

"Hammerhead Squad, come in."

"This is Hammerhead," Darien replied and Amber could hear him fighting to keep control of his voice. "Something else you forgot to tell us?"

"We've never seen anything like it!" Churchwood babbled. "I had the sergeant rewind the footage from Operative Garret's mono-rig. She was confronted by a truly enormous crustacean!"

"I already know that," Amber snapped, unconcerned about leaping the chain of command. "I was there!"

Darien shot her a warning glance and shook his head before continuing into the comm. "You're telling me you've never come across anything like this the whole time you've been camped out up there?"

"No, Operative, we haven't. We've catalogued an impressive array of marine wildlife, crustaceans among them, but nothing of this size. Perhaps it lives in subterranean regions we've yet to reach, or it could be a rare evolution – or even an ancient form of marine life, close to extinction-"

"Professor," Darien interrupted. "Right now I don't care what it is. All I know is that it's blocking our way. The thing has made its home in a passage that we need to swim through to bypass a locked door in order to make our way to the city's control centre."

"It may be large," Churchwood replied. "But it is just an animal – not so far removed from the kind of large crabs and lobsters common to habitable worlds. Its hide may be armoured but it has weak points. The footage shows two large protruding antennae. The creature undoubtedly uses them to navigate and track prey."

"Is he saying what I think he's saying?" Niamh asked, giving Darien a grim look.

"I think so." Darien's jaw tightened as he spoke to the professor again. "So you think we should take it on?"

"I'm not suggesting you try and kill it! I suspect that carapace is more than a match for your lances, but the creature can probably be lured out of your path. It will hunt by sound first and sight second. Create a diversion and simply swim around it. Operative Garret has proved that the micro-jets provide enough force to outrun it."

"Simply. Swim. Round it." Pinching the bridge of his nose, he sighed again. "Alright, Professor, we'll give it a try. Hammerhead out."