I learned some of the acronyms they used to describe certain of these worlds; NTE stood for Near-Total Extinction – meaning bad enough to kill most of humanity, but not all of it. TEA stood for Total-Extinction Alternate. The worst was TPD, or Total Planetary Destruction. Along with these acronyms were further bifurcations of classification, such as Category 3 NTE or Category 5 TEA, terms Yuichi and Nadia tossed around with apparent abandon.

I barely had time to take any of this in before we were once again back in the truck, then back at our arrival point. The air twisted around us, and I felt that same fleeting moment of weightlessness before we materialized somewhere new.

Over the next several hours, I saw things and places I can hardly describe. I no longer had any choice but to accept as absolute truth everything Nadia and Yuichi told me.

The next world we visited looked almost normal, viewed on arrival through the hermetically sealed windows of a vast dome. I saw dusty streets overgrown with weeds and trees, and buildings slowly crumbling from decades of neglect. I saw minarets and onion domes, and learned we were in the outskirts of Istanbul – although not, of course, my Istanbul.

To step outside, I was told, was to die. We were on an Earth that had suffered what Yuichi called a ‘slow apocalypse’. It seemed that decades before, some as-yet-unidentified environmental catastrophe caused the birth rate amongst higher mammalian species first to dwindle, then drop to zero, guaranteeing extinction. Humanity soon suffered the same fate. Then it got worse: everyone under the age of thirty started to die, until the only remaining witnesses to this particular tragedy were a few doddering geriatrics.

The next world proved to be coated in ice to a depth of some miles. After that, we travelled to yet another, with a swollen sun that blazed down on desiccated ruins and oceans reduced to dusty, lifeless bowls.

When finally we returned to the hangar and the island on which it stood, it appeared infinitely more welcoming than it had that same morning.

‘And this place?’ I asked, following Yuichi and Nadia back out through the hangar doors. The sun had crossed the sky, and now dipped towards the ocean. ‘Where does this island fit into your categories?’

‘This alternate is a TEA,’ said Yuichi. ‘Category 1.’

‘That doesn’t tell me anything,’ I said, not quite able to hide my exasperation.