By now we had covered most of the distance across the first vault. The ceiling rose up for perhaps fifteen metres, while all around us stood the empty ruins of buildings constructed within the vault. I had seen maps of the complex’s many levels, and the sheer audacity of the project was breathtaking. They had had room enough down here for a hundred thousand people, nearly all of whom had worked in some capacity towards the construction of the Retreat. There were farms, breeding pens for animals, schools and houses and acres of laboratories. Here, refugee scientists had set up shop so they could help the Icelanders improve and expand their shelter deep beneath the frozen ground. Some of them had been working on creating genetically engineered microscopic flora to manufacture the oxygen they needed to breathe in bulk, down here in the lightless depths.

Nadia tapped with one gloved hand at a dashboard screen and it sprang to life, displaying the view to the rear of our EV. It showed a torrent of white fire gushing through a rent in the wall that hadn’t been there just seconds before.

‘Oh shit,’ she said under her breath. ‘Is that lava?’

I stared at the image, my lips numb even as the glow intensified. It was bright enough now that I could make out street signs as we passed them.

‘How do we get back out?’ I asked, staring in horror at the molten rock flooding across our escape route.

‘We don’t,’ said Nadia, tight lipped and as white as a sheet. ‘Not that way, at least.’

‘But there are other exits,’ I said. ‘Aren’t there?’

‘Yeah, but it’s going to take a while longer to reach them.’

‘Just as long as we’re not trapped down here.’ The steel cage that had wrapped itself around my chest loosened slightly.

‘Go take a look at the maps of the Retreat,’ said Nadia, ‘see which of those exits is nearest. You know where they’re kept?’

‘Sure.’ I walked, stiff-legged with terror, through to the rear of the EV. Barnes, asshole or not, might have been right when he ordered Nadia not to come down here. I pulled open a drawer where printed maps of the entire Retreat were kept, then spread them out on a fold-down table next to the EV’s tiny kitchenette, and tried not to think about the lava slowly filling up the whole vault.

‘We found ’em!’ Nadia practically screamed from up front, just a few minutes later. ‘I got the others on the radio!’