I killed the light and switched on the red laser, dropped to a walking crouch, and carried on, one short step at a time. The wound in my thigh burned from the strain. I moved the rifle barrel from the roof to the floor, the left wall the right wall, cutting a bright crimson sign of the cross in the passageway ahead of me over and over again. It was tiring, disorientating. The laser marker danced in front of my eyes in duplicate, triplicate, so that I had to stop and blink to reduce it to one clear signal. And all the while the roof got lower, and the floor wetter. After barely any distance at all I was ankle deep in water, and getting deeper.

I carried on. The floor began to slope down steeply. Since I’d last been there, nearly three decades before, the muddy puddles my father and I had splashed and waded through had expanded into a full-scale flood. The water reached waist level – which meant, bent over to keep clear of the tunnel roof, I would soon be completely submerged. I combed the surface of the water with the beam of red light. It was crystal clear and smelled slightly of sulphur. The laser cut through it perfectly. There were no ripples ahead of me, and not even a mote of dirt suspended in the stone-filtered deluge. It would be different behind, though. Each step would have stirred up the chalky clay on the still-sloping tunnel floor. I ran the red light down the ceiling until it found the surface. Five metres, after which the passage was completely inundated.

‘Fuck!’ I swore under my breath. ‘What is it with fucking tunnels?’

I’m frequently scared – anyone who tells you they are not afraid in a firefight is either an idiot or a liar. But I have no phobias: no entrenched dread of snakes or spiders, water or flying; no terror in the daytime of the horrors that haunt my dreams at night. But death by drowning was different.

‘Be careful when you go under, Max,’ my father had said to me that night in Paris, suspecting, perhaps, that one day I’d want to try it alone. ‘The surface can be hard to find.’

How I wished it had been my mother he had warned instead.

Drown.

It was a word to gulp down, to choke on; a word it felt like I had been born with under my tongue – tethered to, like my mother had been to the stones in her pockets. I scanned the water for her face, but there’s no use in looking for the dead. I backed up.

Find another way, I told myself.

But I knew there wasn’t – neither in the tunnel nor back in the world. I remembered Colonel Ellard’s mantra, repeated endlessly to any trooper who asked what to do when their officer or their radio died: ‘Always follow the last order.’