We had been walking for two days, surviving on sips of recycled urine and sweat, and drawing sustenance from the high-energy survival tablets in the suits’ equipment pouches. The water was bland and cold and, despite being packed with vitamins, minerals, stimulants and glucose, the tablets weren’t enough to fill our bellies. I had been trying to keep track of the distance we had covered, but had given up some hours back. My best guess was that we had walked somewhere in the region of twenty-three kilometres, give or take.

During all that time, Adam had seldom spoken. He had been quiet since my confession, trying to digest the revelation of my former identity. We walked in silence, but I didn’t mind his reticence; I didn’t want to talk either.

So wrapped up were we in ourselves that we failed to notice the aperture until we were within a few hundred metres of it. We had grown used to the stark, uniform lines of the canyon walls receding away ahead of us like an exercise in perspective. We were mites caught between the pages of a book. Coming suddenly upon a break in this geometric precision jerked us both to a halt.

Adam used a hand to shade his eyes. “What is it?”

The hole was a black rectangle set into the left-hand canyon wall.

“It looks like an opening of some sort.”

“Do you think it’s safe?”

My gaze flickered upwards to the strip of star-speckled night above our heads. “There are degrees of safety.”

“Meaning?”

“We’ve been running for two days. We need somewhere to hide, somewhere to rest and sleep.”

I started walking again, aware that our feet were probably the first human feet ever to have walked the base of this particular canyon, and our eyes maybe the first to behold this aperture. Due to the narrowness of the canyon, it certainly wouldn’t have been visible from orbit, and ground expeditions were strictly prohibited.

By the time I reached the foot of the wall beneath the opening, I realised my sense of scale had been way off. It was difficult to judge the size of things against the two-kilometre vastness of the slab-sided canyon walls. The hole, which I had initially thought to be fairly small and close to the ground, turned out to be at least two metres in height and five in width, and almost two metres above the ground.

A pattering noise echoed up the canyon from the direction in which we’d come. For a moment, it sounded like distant rain. Then I recognised it. Hundreds of steel claws were skittering towards us.

“They’ve deployed crawlers,” I said.

“What?”

“Drones shaped like millipedes. They’ve probably released a swarm of them into the labyrinth.”

“Are they dangerous?”

“Lethal.” I glanced up at the hole above our heads. “But if we can get in there, we might be able to hide.”