The heavy gunmetal-grey door activated automatically on their approach along the wide causeway that stretched out from the coast, swinging open with a hiss and closing behind them with a loud clang. It was the four of them; Keri, Cassandra, Anisa, and Andreas. Eu had stayed behind with the van, in case of... well, just in case.

The second door opened seconds after the first closed; there would be no major pressure gradient to worry about at this shallow depth, at least.

The hallway they entered was dark and deserted, the only light provided by their torches and some weak sunlight shining fitfully through the dirty acrylic glass and the polluted waters beyond. The walls curved outwards, creating a squat, round passageway that extended into the darkness. Several branches were barely visible further ahead, and the arrhythmic sound of dripping water echoed from their depths.

"Just condensation," said Andreas firmly, in response to the as yet unspoken uneasiness the sound induced. He hesitated. "Or something."

Keri hoped he was right. Despite the chill, the air was unexpectedly damp. Their footsteps splashed in a thin layer of moisture that covered much of the floor.

They hadn't been sure what the atmosphere would be like down here, but it seemed no more than a little stale. An emergency rebreather hung from a small pack on Keri’s back, ready at a moment's notice. The others each had one too.

"Looks like nobody's home," said Cassandra, loud enough to give the others a start.

"Someone's here," said Andreas.

The rest of the group looked to where he was pointing. A camera in the centre of the roof was turning to face them, green LED blinking below the lens.

"Well, it'd be nice if someone came to greet us," said Cassandra, directly at the camera. "We came all this way, after all."

The lens stayed frozen on them, light flashing malevolently.

They wandered deeper into the dark.

"The size of this place!" said Anisa with a whistle as they stepped through another airlock.

The passages were wider here, punctuated by huge caverns set out for different purposes. Each cavern was joined to the next by rows of tube-shaped corridors, spread across several levels through the ocean that flowed between. What had seemed to be a solid sphere from above turned out to be a collection of titanic metallic pods formed into that shape, never quite meeting but locked together through connecting passageways.

They wandered deeper still.

They moved through empty entertainment complexes filled with the games of yesteryear, garish colours rendered dull and lifeless by the unending damp. The machines lay, silent and inert, inoperable after decades of exposure to the moisture-filled atmosphere.

They wandered through plazas and parks obviously designed to reflect the wildernesses of the world above, despite the fact that even when Triton was yet young those wildernesses were more a memory than a reality. The strip lighting that would have provided the UV necessary to promote growth in such an artificial setting hung loosely from fittings high above, extinguished for eternity, and below lay rotted vegetation, knotted and moulding in brown piles of dead flowers and fallen trees. Without soil management and careful climate control these parks must have quickly died, and the air stank of damp and organic decay. Ironically, the driest places they had found so far were here, in the empty ponds and streams, left desiccated when the water filtration and purification devices that maintained them broke down.

They wandered through the domestic blocks. Keri stopped to look at some of the living quarters, lined in row after row above one another, stretching into the gloom overhead. Even the smallest of these was at least twice as wide as the average modern habitat, and most consisted of two or even three floors, jutting out from the walls of the structure and into the ocean beyond, their few small circular windows showing only darkness flecked with brown dirt, but providing what must have been an endlessly-fascinating view when the water had been clear and the outside lights active. Several of the apartments had actual wooden furniture, something you almost never saw these days, stained and falling apart in the damp.

There were few cameras they could see, but the ones they spotted inevitably swivelled to follow them.

Keri felt her ears pop. They were getting deep now, deeper than she had hoped they would have to go, but no matter how many vestiges of past times they found, they saw no sign of recent activity. Viewports, too, became smaller and fewer, aesthetic priorities giving way to the reality of building on such a grand scale at such a depth.

But the light remained. Power extended deep into the facility.

It took at least a couple of hours to find their way to the colossal airlock the led to the second section. It towered over them, a huge industrial wheel several stories high, ringed with cogs on which to roll aside when necessary.

"Why so big? Butchers could turn themselves into giants or something?" said Cassandra.

She had meant it to be a bad joke, perhaps, but the encroaching darkness and alien environs sucked any trace of humour from her words. Images of mechanoid titans forcing the door aside with colossal metal claws flickered through Keri's mind, unsettling for all their ridiculousness.

"It makes sense," said Anisa, uncertainly. "They'd have to use some heavy equipment to make all this. Gotta get it through somehow."

"Hmm," said Andreas. He didn't sound so certain.

The giant airlock felt out of place, entirely impractical and echoing a style of design at odds with the rest of the facility, seeming not only like a different generation of technology but a different branch, a steampunk dream of an alternate reality. It didn't seem real.

They used a regular airlock, set a short ways from the larger one, an airlock the same in every way as the ones they had used before. It seemed incongruous, but worked just fine. They gathered in its central chamber as air was forced in, equalising with the pressure maintained beyond.

When the lock swung open this time, Kilgore was right there.