Wet blackness engulfed me. The last pockets of air in my clothing bubbled out. I trod gingerly, left hand in constant contact with the wall, M4 braced into my right hip, barrel forward, describing the same hypnotic red cross on the passage ahead. Progress was painfully slow. I stubbed my toes on hidden rocks, scraped my skull on the invisible ceiling. One minute in and I’d covered less than ten metres. After a minute and a half I reached another T-junction. To the left the red beam vanished into the clear black water. To the right the route curved around. I followed it, left arm out, steadying myself in the weird weightlessness of the submerged passage. The route divided again. I kept right, completing a hundred-and-thirty-degree turn back on my original direction. If I’d worked it out correctly, I’d be facing south-west. I stood still in the blackness. My two minutes were up. I began to feel the tightness in my chest. The straining at my throat.

If I went back now, I’d have to risk going all the way back, and that risked a lot. There could have been five metres or fifty metres of flooded tunnel still left to navigate – all with unknown height and access. There was only one thing to do: go white again. The tactical LED on the M4 would illuminate the water like a floodlit swimming pool. I’d be able to see if there was a way out. But anyone on the other side would see me coming out of the dark long before I saw them.

Lungs pulling in my chest, I weighed up the pros and cons and decided to go white. Sometimes you doubt if you’re making the right decision. But whenever there’s doubt, there is no doubt. I let a little air out of my lungs, and found the on switch for the torch with my left thumb. As I felt the pad begin to depress, the water ahead of me glowed with a stream of bright white light.

I could see, immediately, that I was standing in a tangle of femurs and cracked skulls, washed into the passage with the floodwater. Their jagged shadows leapt towards me. But my thumb hadn’t made full contact with the switch. The light wasn’t coming from me. It was coming at me. I could see, too, that the floor of the still completely submerged passage ahead began to rise, ending in a pool only a few metres away, where the tunnel climbed up sharply over a rockfall out of the water. I resisted the strong urge to exhale and sank down into the bones, trying to make out what was happening beyond the waterline.