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Ifind the first of Vane's devices eleven meters down.

It's a limpet relay, magnetic, no larger than my thumb, clamped to the shaft wall behind a lip of stone where a diver descending in a hurry would never look. It is nine months old at most. Its battery is good for two years.

I take it off and crush it, and then I stop swimming and think about what it means, and what it amounts to is not good.

Vane has had a man down this shaft.

Not into the observatory — he'd have taken it apart already if he'd got inside. But somebody worked out where the entrance was, came down here, planted a listener, and went home. Which means Vane has known the location for most of a year and has been waiting, not searching, and the one thing he lacked was the sequence.

He didn't need the ninth artifact to find the door. He needed it to open one.

I find six more relays going down. I destroy all of them, which will tell him precisely as much as leaving them would, and considerably faster, and I decide I want him to know we're here. A man who has been patient for eleven years makes his worst decisions in the four hours after patience stops working.

At two-sixty the shaft ends in an airlock, and the airlock still functions.

That is the moment the expedition stops being an expedition.

Selka goes through the cycle ahead of me, and I hear her through the water before the inner door has finished opening, which for Selka is the equivalent of shouting.

Then we are all standing in a corridor with air in it.

Ten-thousand-year-old air, filtered by something still running, at a pressure my lungs accept without argument. The floor is dry. The walls carry a soft light with no fixture I can identify. Behind us, twenty-two people's worth of dripping.

Joren says a word I won't record.

“Agreed,” says Selka.

The corridor runs eighty meters and opens, and I lack useful language for what it opens into.

Call it a chamber. Call it a cathedral raised by people who had no gods and unlimited time. It is four hundred meters across and I cannot find the ceiling with a lamp. Suspended in the volume of it, from nothing, at every height, are instruments — rings inside rings inside rings, some no bigger than a boat and some the size of this village, turning at speeds so different that the small ones blur and the large ones appear entirely still until you watch them for a minute and understand that they are not.

All of it is moving. None of it has stopped in a hundred centuries.

“They're armillaries,” Selka says. She has both hands flat against a rail and she is not entirely steady on her feet. “Draeven. They're armillary instruments. That's a design so simple that eleven separate species invented it independently, and these have been running since before my species had a word for a year.”

“What are they tracking?”

“Everything. Look at the declination on the outer ring — that's not a local sky. That's not Castus. That's galactic.” She laughs, badly. “They're not observing. They stopped observing when the makers left. They're keeping. They've been keeping the position of everything they ever catalogued, updating it continuously across a hundred centuries, against the day somebody came back and wanted to know where things had got to.”

I take four men and clear the perimeter, because it's the job and because if I stand still and look at the rings for much longer I am going to be no use to anyone.

Here is what ninety minutes of methodical search turns up, and it changes my professional opinion of this entire operation.

There are nineteen chambers off the main volume. All nineteen store. Records, in a dozen formats, most of which Selka cannot identify and two of which she can. Physical samples in sealed cases. A hall whose walls are one continuous inscription from floor to unreachable ceiling.

There are no weapons. There is no reactor I can locate, no defensive emplacement, no armory, no vault, nothing that could be sold as an application to anybody. There is not even a door that locks from the inside.

Whoever built this did not build it to survive an attack. They built it to survive time, which is a longer war and a stranger one, and they clearly never imagined that the first people through the airlock might arrive with cutting gear.

I go back to the main chamber to report and find Clara sixty meters up.

There's a spiral of platforms winding around the inner wall, and she has climbed two hundred steps of it with Selka's slate under one arm, and she is standing at the rail with an entire artificial sky turning slowly beneath her, and she has forgotten I exist.

I stop where I am.

Assessing risk in real time is a trained reflex after nineteen years and I do not get to switch it off, and what it is telling me, standing at the foot of that spiral, is this: seven boats are roughly two hours behind us. There is one exit. There is no defensible line between the airlock and this chamber. My professional advice, if anyone asked for it, would be that we photograph what we can, transmit what we can, and be four kilometers north before Vane's first man reaches the shelf.

That is the correct advice. I don't intend to give it.