The room was a sphere, about fifteen spans across, and the skull was floating in the middle of it like the main exhibit in an art gallery. It was trussed up in a kind of bridle, a frame made up of metal bars and struts, and this bridle was fixed onto the walls by dozens of springs.

‘Quieter and stiller we hold her, the better,’ Cazaray said. ‘Trig’s damped the ions, and that helps, but it would only take a jolt to rattle something loose.’

From end to end the skull was about as long as my sister was tall – about eight spans. It was the colour of a bad tooth, rotten to the root. It wasn’t anything like a monkey skull. It was stretched out, all snout and jaw, more as if it had belonged to some giant horse than a person. It was made up of lots of parts, joined together like a sort of puzzle. A dark fissure ran across it, stitched together with a ladder of little metal sutures. Whoever had done that had worked a long time ago, and with care.

The skull had also been drilled and tapped in many places. Slender probes and wires had been pushed through the bone into the not-quite-emptiness within.

‘Tell me what you know,’ Cazaray said, never raising his voice above a murmur.

‘It’s old,’ I answered.

‘How old?’

‘No one knows,’ Adrana said.

‘Good answer. And it’s the truth, too. They were finding skulls in the Sixth and Seventh Occupations, but whoever left them must have been through these parts a lot earlier than that. They don’t fit the morphology of any of the aliens we know about today, not the Crawlies or the Stingtails or the Tuskers. Some people who ought to know better think they come from dead Bonies or Bug-Eyes, but I’ve seen enough of them to know that can’t be true. My guess is that the coves who used to own these bones died long before people got going. It was some other aliens who left the skulls here, and they used them just the way we do, like a kind of squawk.’

From opposite sides we peered into its mysteries.

‘It’s not empty,’ Adrana said. ‘There are little lights, flickering on and off.’