‘No, no,’ I told him. ‘This is the one they want. I’m sure of that. We’ve got to crack it. What we need to get through this lot is breathing gear and dry-suits.’

‘Yeah. But how do we know what happens the other side of the water? If there is another side. If the rest of the tunnel’s flooded we’re buggered. Jesus, I hate this!’

‘It must be quite a small leak,’ I said. ‘Otherwise, like you said, the whole tunnel would be full. Maybe the pressure’s equalised itself somehow – or mud’s filtered into the fissure.’

‘Let’s get the hell out, anyway.’

I’d been planning to sweep away our footprints behind us, but I realised now that, even if we went to that trouble, we’d still leave fresh marks and it would be obvious that somebody had been down here. In any case, the chances of anyone else coming down in the next few days seemed infinitesimal.

We were back under the access shaft just twelve minutes after leaving it. Eighteen minutes to wait. I tried the radio again but got no response. I wasn’t going to shout, just in case some Russian was passing the old stable up top. I imagined Toad, on the lurk up there, and Whinger, on standby in the Volga somewhere along the embankment. Maybe they were chatting to each other on the radio.

‘Have to wait,’ I whispered. ‘Let’s take a stroll in the other direction.’

That didn’t get us far. This time I wasn’t counting the steps, but about a hundred metres to the south the tunnel was blocked by a major fall. The damage to the roof and walls was so extensive that I felt sure they’d been bulldozed in or deliberately dropped by hand. Bricks, rubble and clay were tumbled in an impenetrable mass.

Back under the shaft, we waited. We peeled off our sodden overalls, but still we were soaked to the waist and higher. Soon we were pretty cold. I went over the various levels of our fall-back plan in my mind. The first was that if Toad got accosted in the yard, he’d pretend he was drunk and had staggered in there to sleep it off. The next level was that if we three didn’t reappear, Whinger would park the car out of the way and come looking for us. The final stage laid down that if all four of us weren’t back in camp by 6.00 a.m., the rest of the team would come out to search. I knew that in an emergency we could seek sanctuary in the grounds of the Embassy, but that could only be a last resort because it would blow the whole Apple programme.