Nadia took us around the edge of the abyssal shaft, and before long we were rolling down the steep-angled road that looped around the edges of the shaft. Some of the mountains on the outskirts of Reykjavik’s ruins were, in fact, nothing more than mounds of excavated dirt. On the way down we passed numerous abandoned vehicles that had been used during the excavation.

It took a full twenty minutes before we finally reached the bottom of the shaft and the first enormous door, designed to seal the city within from the harsh environment without. Someone had used nukes to blow through those doors – the Americans, perhaps, or the Russians, or possibly even some other faction within the multilateral navy whose ships were still moored in the frozen sea off the coast.

This hadn’t worked out well for the invaders, or the people they were invading. From the presence of their tanks, armoured cars and other weapons of war, it was clear that this army of nations had succeeded in taking the caverns from the native Icelanders, killing a substantial portion of their population in the process. Then, so far as anyone could tell, they had set about killing each other. In the process, they had done sufficient damage to the Retreat to ensure that no one was going to survive the big freeze.

We trundled on down the sloping floor of a long tunnel. The twin cones of light from our headlights were sharp edged in the vacuum. Soon the tunnel widened into an enormous cavern, its walls and roof invisible beyond the beams of light. I knew that the buildings and living spaces all through the vaults were filled with frozen corpses in their tens of thousands, and the thought of so many dead – and so many ghosts – made my skin prickle with horror.

I peered nervously into the darkness beyond the reach of the headlights. ‘So what is it particularly the Authority were looking for on this alternate?’ I asked. ‘Did they tell you?’

‘Officially,’ she said, ‘all I know is that they’re looking for scientific data of some kind.’

I looked at her. ‘“Officially”?’

She gave me a sly grin. ‘I’ll get to that. See, back when they started building their underground Retreat, the Icelanders on this alternate had the bright idea of inviting a bunch of really smart people from other parts of the world to come live in it with them. After Shiva showed up in their telescopes, a lot of people around the world decided to blame the scientists, as if they’d somehow caused it. Most of the very people who just might have been able to figure some way out of the mess they were all in wound up being hanged or burned alive on the grounds of universities.’

‘Delightful.’