We had marched for most of an hour when one of the technical crew, consulting an instrument, called a halt.

“This is the perimeter,” Oscar whispered: the point of proximity beyond which all pilotless drones had mysteriously failed.

Three of the soldiers marched ahead while the rest of us waited nervously. The snow had thinned and there were open patches of sky above us, but daylight was fading fast. The science crew aimed a couple of their lights into the gloom.

The point men halted at a fixed distance, then waved us on. We followed from a prudent distance, announced by sweeping beams of light—we would be hard to miss, I thought, if the Hypotheticals happened to be looking.

But we were well inside the perimeter now, and nothing had happened.

*   *   *

The temperature dropped with the fall of night. We cinched the hoods of our survival gear tight around our face masks. The wind remained brisk but the squalling snow stopped suddenly, and in the clear air we could make out the shapes of the Hypothetical machines ahead of us, startlingly close. The technicians hurried to aim their mobile lamps.

We had been calling these structures “the Hypothetical machines,” but from the ground they looked less like machines than huge geometric solids. The nearest of them was a perfectly rectangular cube, half a mile on a side and moving at a slow but (barely) perceptible speed. Now that we were close to it I believed I could feel that ponderous motion under my feet, a gentle seismic tremor.

We approached the cube in silence. The soldiers on point were dwarfed by it. The technicians began to angle up their lamps, playing the beams against the nearest vertical face, a featureless surface the texture of sandstone. Because of its regularity it was hard not to think of this thing as an absurdly large building, but it was a building without windows or doors, as enigmatic as a sealed pyramid.

For a while we did nothing but stare at it. Oscar said it must already have detected our presence; but if it did, it failed to react in any obvious way. Then the technical crew got down to business. They erected tripods and secured their lamps on them; they unpacked sensors and recording devices and anchored them in the pebbly, cold soil. A steadily increasing number of fiercely bright beams divided the desert into a quilt of light and dark.