I’d decided in advance that we weren’t going to ponce about mortaring over cracks in the brickwork. The chances of somebody else reaching the site were remote – and anyway, new mortar wouldn’t pass a close inspection. Now that Apple was live, I wanted to get the hell out of the tunnel as soon as possible. So we simply covered the casing with a loose mound of bricks and spoil, as though the heap had fallen from the roof, and pushed some lumps into the conduit that we’d cut for the connection, to hold the cables in the duct. Then we collected up our kit and prepared to withdraw.

‘Toad,’ I said, ‘what happens if the water level comes right up and the thing gets flooded?’

‘It shouldn’t make any difference. Now the units are sealed together they’re waterproof. There’d be problems if the level got as high as the SCR, but I don’t reckon that’s possible.’

There was one last precaution I’d decided was worthwhile. Back at the edge of the water, we used one of the empty rubber bags as a water carrier, filled it, and dragged it to the base of the blocking wall. There we tipped the lot out at once, retreating backwards before a little tide that pursued us down the tunnel. By doing that four times, we washed away every sign of disturbance and left the silt on the floor in a smooth, unbroken carpet.

Then we waded away through the flood.

We were back under the shaft by 0020. We’d missed the midnight rendezvous, but in only ten minutes Rick was due to make his next inspection. Our last batteries were all but spent. As we waited in pitch blackness, my mind wouldn’t leave the twinned cases, buried under the mound across the river. I thought of the device as a time-bomb, ticking away towards detonation. I knew that wasn’t how it worked, but the idea wouldn’t fade. How could we be sure that some idiot in the Pentagon wouldn’t set it off by mistake? We had only Toad’s word to give us hope that accidents were impossible.

We waited, sweat congealing, grit itching inside our shirts. I found myself thinking of the occasion, years before, when we’d buried an old aunt in the churchyard of my village, in the north of England, how the clods of earth had rained down on her coffin as the gravediggers started to fill the hole above her. There was something uncomfortably similar about the way we’d heaped the spoil back on top of Apple’s black and green casing.

On the dot of 0030 we heard a creak of hinges above us, and a beam of light flickered down the shaft.

‘Anyone for the up?’ Rick called softly.

‘Three,’ I told him. ‘Can’t wait to get out. Everything OK on top?’

‘Fine.’