It seemed to Jeff that the more intelligent people were, the harder it was for them to deal with witnessing a darkened universe far advanced in its long, slow senescence. Self-declared atheists began sporting prayer beads, while the moderately religious either discovered a new fervour for their faith or, more frequently, abandoned it altogether.

Farad refocused on him after a moment, and Jeff could see that his face was slick and damp behind the visor.

‘I’m sorry,’ said Farad after a moment. ‘Sometimes . . .’

‘I know,’ Jeff replied, with as much sympathy as he could muster. ‘But we’ll be home in a few days. Remember, we’ve got a plan.’

‘Yes.’ Farad nodded, his upper lip moist. ‘A plan. Of course.’

‘You just need to hold it together for a little while longer. Okay?’

‘Yes,’ Farad said again, and Jeff could sense he was a little calmer. ‘You’re right. I’m sorry.’

<

Jeff gave him his best winning smile. ‘You already said that.’ He nodded, indicating somewhere further up the corridor. ‘I think we’d better catch up.’

Eliza had glanced back once, but chose to say nothing as the two of them caught up.

They pulled a spare tokamak fusion unit from a pre-fab warehouse established in Vault Four’s primary atrium and loaded it on to the rear of an electric cart, before letting it zip them up a steep incline that switched back and forth the higher they rose. When they reached Level 214, they found the passageways and chambers shrouded in darkness, so had to rely on their suit lights while they swapped the new fusion unit for the failed one. There was no telling why it had shut down, but inexplicable power-outs were far from unusual.

The lights strung along the ceiling flickered back into life, revealing closely cramped walls on either side. An airlock seal had been placed across the passageway, and they stepped through it one by one, emerging into the pressurized area beyond.

Jeff wanted nothing more than to crack open his helmet and breathe air that didn’t taste like his own armpits, but Eliza would have none of it. He understood the reasons for her justifiable caution, but still felt resentful.

When they entered the chamber that Stone’s team had been studying, they found Dan had now managed to drag Lucy on to a narrow strip of ground located between four adjacent pits. The pit that had swallowed up Stone and Vogel was now full to the brim with black oil, its calm stillness looking to Jeff like a black mirror laid flat on the ground. It seemed strange that none of the reconnaissance probes first sent into this chamber had triggered a similar reaction.