Amber regarded the chasm grimly. She joined the others at the edge, dropped to one knee and shone the light of her gun-mounted torch down into the darkness where it was simply swallowed up. Beside her, Niamh twisted a glow-node into life and tossed it into the pit. It fell for almost a full minute, its spherical mass sending out light in all directions, when suddenly it struck something and vanished.

Seconds later the faintest echo of a splash reached her ears.

"Water," Amber said. "It must be flooded down there."

"That's a long drop." Niamh shook her head and held her mapper out over the hole, firing a pulse down into the pit. She let out an impressed whistle. "One thousand three hundred meters long, to be exact."

Amber glanced around. The gap wasn't really a problem in itself. She could have Blinked the distance with her eyes closed but her real concern lay with the structural integrity of the remaining corridor. The underwater city was one of the most formidable buildings she'd ever seen and it looked more or less intact. Maybe the corridor had collapsed over the long march of time, but something didn't seem right. She shone her light up the walls around the chasm and saw unmistakable cracks and fissures; time had not done that.

"Professor Churchwood didn't say anything about seismic activity, did he?" she asked.

Darien shook his head. "Not that I remember. Why?"

"Because that looks like seismic stress damage."

"Could have happened hundreds of years ago," he replied. "C'mon, we can still cross." He pointed to a spur of unbroken flooring that protruded a ragged two metres from the far right hand wall, forming a thin bridge from one side to the other. "Fall in, folks. We're not here to gawp at empty spaces."

Amber moved up behind Idas as the operatives formed into single file on the narrow ledge. Behind her, last in line, Brannigan stepped into place, eyes darting back and forth between the chasm and the ledge.

"You good?" she asked quietly. The new girl nodded, her previous bravado apparently driven away by the sight of the kilometre deep pit. "Stay close."

"I will." Brannigan nodded.

They walked slowly and carefully down what remained of the corridor, and Amber could see the jutting cragged edge where the rest of the floor had fallen away. Where the huge blocks had been split apart a silvery sheen was revealed, gleaming under her torch light. The material seemed to be some kind of hybrid of rock and metal.

"Darien," she said quietly, her voice rising above their footsteps nonetheless. "Hold up."

"What is it?"

"We should get a sample of the interior of these building blocks," Amber elaborated. "We don't have the gear to crack the material ourselves. Might as well take what we're given."

He considered this for an instant then gave her a nod. "Do it."

She dropped into a crouch, freeing a siphon-module from her backpack. A small tube barely a centimetre in diameter, the little tool buzzed faintly when she pressed its tip against the shining stonework. Thousands of microscopic drill-bits dug into the strange metallic stone at a molecular level. For the few seconds it took to gather a sample Amber tried not to look down.

She straightened up once the task was complete, securing the tube in her combat vest and stepping gratefully away from the edge. Looking up the line she made a chopping gesture toward Darien, indicating they should keep moving. They pressed on, cautiously negotiating the gloom until they were about halfway across the chasm, when a low rumble reverberated through the caverns of the city.

Amber stopped in her tracks, feeling her body tense up. "What was that?!"

No-one replied at first as every operative halted and listened. A few seconds later another rumble sounded somewhere in the depths and this time she felt a tiny but definite tremor pass under her feet. She looked down at the floor, eyes wide.

"What was that you said about seismic activity?" Idas hissed.

"Double time!" Darien barked. "On me!"

The group began moving at a jog down the narrow ledge as another rumble passed through the walls and the tremors visibly shook the walls, dislodging small clouds of ancient debris. Amber stumbled, feeling a surge of vertigo as she lurched toward the black pit on her left. She shifted her weight instantly, adjusting for the shift of the ground and bringing herself back into the middle of their narrow path.

At the head of the group, Darien was barely ten metres from the far side of the chasm when the full impact of the seismic force passing through the city hit their current position. The very ground beneath their feet spasmed violently and the groan of the superstructures filled the air. But amid it all, another sound penetrated Amber's hearing: a high-pitched cry from behind her.

She reacted instinctively, moving even before the echo of Brannigan's panicked yelp had died. She heard the slip of the other girl's boot against the damp stone and the scrape of fabric as she lost purchase. Sure enough, she pivoted to see their new team mate pitching forward as the stone gave way beneath her, with nothing but inky blackness waiting below.

Without a second thought, Amber lunged forward and threw out an arm, catching Brannigan by the scruff of her combat vest as the girl toppled over the precipice. As she did, she also flung her free hand back behind her. Brannigan's weight jerked her forward off balance and there was an instant of horror when she felt herself teeter on the brink, pulled by the downward momentum. Then she felt an iron grip clamp down around the wrist of her other arm, sharply halting her plunge into the depths. Her trust in Idas had been well-placed.

"Oh my God!" Brannigan gasped. "Pull me up! Pull me up!"

"I've got you," Amber said through gritted teeth. "Just stay still; stay calm." Even as she spoke she could feel her fingers slipping. Brannigan's bodyweight wouldn't have presented a problem, but the extra thirty pounds of non-standard equipment left her struggling to keep her comrade aloft.

"Stay still," she repeated. "Idas, can you...?"

"Yeah, I gotcha," Idas said. "Just hang onto her."

She felt his other hand latch onto her collar and slowly, carefully, he levered her back up into position on the narrow ledge. Now with her feet firmly planted, Amber was able to free her other hand. While Idas kept her anchored in place she reached down and took a firm grip on either side of Brannigan's combat vest and, with a growl of exertion, she hauled the other girl back up. They stepped away from the edge, but Amber kept a hold of her companion, seeing the she was still staring down into the blackness.

Brannigan swallowed a deep lungful of air and tore her gaze away. "Thanks," she stammered. "I didn't...the ground just gave after that quake-,"

"I know," Amber said quickly. "It's not your fault." She looked up the line to where Darien waited, watching with concern stamped on his face. "We're fine," she said. "Let's just get back onto solid ground and radio the nutty professor. He's got some questions to answer."

"You're damn right," Idas rumbled.

The operatives scrambled over to the relative safety of the adjoining passage, where the stone floor remained fully intact and the lights were working once again. Even still, they gave the edge a wide berth as the seismic rumbles continued, receding now into the distance. When they stopped, Brannigan sank down into a crouching position, looking fearfully back over her shoulder at the darkness. Amber dropped down beside her, putting a hand on her shoulder.

"Are you okay?"

"Yeah, yeah, I'm fine," Brannigan answered. She pressed her lips tightly together and inhaled another steadying breath. "It's just, this is my first mission and that – that could've been it, y'know?"

"I know what you mean." Amber nodded. "The important thing is that it wasn't. You're still here, operative."

Further conversation was interrupted as Darien's furious voice went scything through the comm channels. "Hammerhead to all teams, we've just experienced a serious seismic tremor in our part of the city. Give me a status report right now!"

Mina replied first, and Amber could her the tension in her voice. "This is Vandal. You're not alone Darien. That tremor caved the roof in on the room we were searching!"

"Casualties?"

"Nothing serious," she said icily. "A couple of scrapes and bruises, nothing we can't fix ourselves, but we were lucky."

"Tundra?"

"Tundra here. I think you guys got the worst of it," Vass answered, and even he sounded subdued after the quake. "We felt the tremors but the full force never impacted here. No casualties. Structural integrity of the dock seems intact."

"Copy that," Darien replied. "Everyone stay sharp; looks like we're already picking up more than we bargained for down here. Proceed on mission as planned but exercise extreme caution."

One the other squad leaders acknowledged, Darien flicked through the comm channel again. Amber changed her frequency too, listening as her commander gave vent to his feelings at their comrades back on the sub.

"Hammerhead to Manitta-Vanna, come in."

"We read you, Hammerhead," Link answered straight away. "Is everything okay down there? Our sensors just recorded some kind of tremor under the main superstructure."

"It was more than a tremor, Sergeant!" Darien exploded. "I nearly lost one of my operatives over the edge of a cliff. What the hell is going on up there? Which slate-brained space-walker decided to leave earthquakes out of the operational briefing?!"

"Watch your tone, operative," Lieutenant Tyndall shot back. "We didn't include it because there was no seismic activity to include. This hasn't happened before."

Professor Churchwood's exuberant voice burst over the comm next, "Darien – operative – the lieutenant is being utterly candid in this regard. The city was constructed here because the surrounding area is one of the most stable regions on the planet. We haven't recorded any seismic activity in our entire year of observation."

"Come down here and say that to me," Darien replied, shaking his head in disbelief. "I know what an earthquake feels like, Professor."

"It's not so simple," Churchwood said. "You may have felt it, but our readings show no actual shift in the tectonics. We registered the shockwave, but according to our instruments there hasn't been any movement in the plates. There shouldn't have been a quake at all."

Darien's brow furrowed. "Then what the hell happened?"

"I'm afraid we're not sure yet. My people are analysing the data as we speak. I'll pass along any information we find, you have my word."

"Be sure that you do. Hammerhead out."

Amber could see anger and confusion vying for control of Darien's features. She stood up, resting her carbine across one shoulder, waiting expectantly for his orders. Eventually he exhaled a long, calming breath and motioned down the corridor with a flick of his head.

"A little shaking isn't going to scare us off," he declared. "We go on."

And on they went. Being back under the pyramidal lighting nodes gave Amber a feeling of reassurance. Alien though they may be, they showed that the structural integrity of the place was sound – the quake hadn't caused any more damage to their section. On top of that, she spotted their destination: a door that loomed over them like some kind of primeval monolith.

Built of a darker material than the surrounding walls, it was triangular in shape and stood an impressive thirty feet tall. The two symmetrical slabs that made up the door formed a huge picture and she raised an eyebrow at what it displayed. This was the first image carved into the structures that she could even vaguely recognise. She didn't like what she saw.

The mural showed some kind of creature – a spider-like thing with a bulbous, ovular body and a dozen slender legs arranged symmetrically down either side. The legs ended with wicked hook shapes, but it was the carving of its head that really made Amber uneasy. The sculptor had clearly set out to exaggerate the full horror of the thing, carving a forest of eyes into an enormous skull, and giving it a jaw that gaped with long, dagger-shaped teeth. Cut into the door around the hideous visage were more of the geometric symbols and lines, arranged in a perfect ring.

"That's delightful," Niamh muttered, shining her torch over it.

"Never mind the aesthetics," Darien replied. He pointed to the panel on the far right of the colossal structure. "Brannigan, Amber – check the controls."

Amber led the rookie operative over to the panel, but at a glance she didn't think there would be much they could do. The buttons – if that's what they were – were labelled with the same unintelligible script as everything else. Not only that, but there didn't seem to be any power feeding into the dark plate of metal despite the fact that the lights overhead worked.

"What do you think?" Brannigan asked quietly.

"I think I don't want to start smashing buttons if I can avoid it," Amber replied. Carelessness was not in her nature. She peered more closely at the arrangement. A central square rose higher than the other shapes – this one marked with two half-triangles separated by a sliver of space. "If I had to guess I'd say that's the door release, but doesn't seem like the panel's online."

Brannigan reached forward and pressed the button. Amber looked at her sharply but the girl's face didn't waver. Her action accomplished nothing regardless. The door remained closed.

"Controls are dead," Amber called to the others. The door presented a formidable obstacle, even to them. Blinking to the other side would have been easy, but with the mapper unable to penetrate the stone-metal alloy they would have no idea what was waiting. It could have been sealed to hold in flood water, or for all they knew there was a sheer drop on the other side.

Hekket pointed down an adjoining passage on their left. "There is another stairwell that leads down to a parallel passage thirty meters that way. We might be able to find a way around."

The operatives filed through into the tangent hallway and it didn't take long for them to find the passage in question. Darien stopped, stared, and let out a quiet curse. Amber pulled up alongside him, saw what he saw, and could only nod in agreement.

Hekket's directions were correct. There was indeed a stairwell, and it did lead down to a parallel corridor. The only problem was, it was underwater.