Rodriguez’s team-mates, when they finally found him, had been a lot more cautious. One had thrown a spanner just to one side of Rodriguez’s frozen figure, from the safety of the chamber entrance. It still hung there now, motionless, caught in the course of its long trajectory through the air, on its way to eventually landing in some future century. The reconnaissance probe – a wheeled platform mounted with cameras and a range of sensitive instrumentation – stood equally immobile nearby.

David Rodriguez, as new recruits to the most secretive department of the UW’s retrieval and assessment bureau were told, had been a fucking idiot. The vaults were filled with unpredictable dangers, which was why they had to stick to the paths already pioneered by the probes. You wandered away from them at your own risk.

The current popular theory was that these slow-time chambers were stasis devices designed for long-term storage. Time-lapse cameras had been set up at the entrance, to try to estimate how long it would take Rodriguez to set his right foot down, turn around and walk back out of the chamber. The best estimates suggested anything up to a thousand years.

Sometimes Jeff woke from nightmares of Rodriguez still standing there, his face turned away, as the years turned into centuries. Sometimes he was Rodriguez, waking to find himself lost in the darkness of some future age, all alone on the wrong side of a wormhole gate that bored its way through time and space very nearly to the end of everything – a hundred trillion years into a future where most stars had turned to ashes, and the skies were filled with the corpses of galaxies.

They re-emerged from Vault One and followed the North Rampart until they reached Vault Four, half an hour after receiving Dan’s distress call.

Beyond the vaults lay nothing but the blasted, airless landscape of a world that had been dead for immeasurable eons. The planet on which the vaults stood orbited a black dwarf: the shrunken, frozen remnant of a once bright and burning star whose furious death had long since stripped away any vestiges of atmosphere.

Dan, who was an expert in such things, had once told Jeff the vaults themselves were tens of billions of years old, meaning they had stood for longer than the entire lifespan of the universe as it had been measured back in their own time. They were constructed, too, from a material that resisted all attempts at analysis. Despite a near-eternity of bombardment by micrometeorites and other debris drawn into the planet’s gravity well, the exterior of the vaults appeared as smooth and pristine as if their construction had just been finished.