‘Get hold of yourself!’ I snapped. ‘We haven’t got time to piss about.’ In my mind I added, A great big feller like you, too! But I knew his hang-up was getting to him.

A few seconds later he said unsteadily, ‘I’m all right now.’

‘Come on, then.’

I moved forward, counting. For 102 paces the floor of the tunnel remained level. Then it began to descend.

‘Going down under the river,’ I said.

‘Aye,’ Pavarotti agreed. ‘I reckon.’

At the start of the slope was a big heap of debris. Such a chunk had fallen out of the upper right-hand wall and roof that the pile of bricks stretched across the tunnel floor to the base of the opposite wall, and we had to scramble over the lowest part of it. When I directed my head-lamp at the raw wall where the bricks had been, I saw that it consisted of moist grey clay.

‘At least we can dig into that,’ I muttered.

‘Pity we can’t put the bloody thing in right here. Save messing about.’

‘It’s too far from the proper site.’

We were still talking in whispers, partly out of habit, partly because we reckoned any sudden noise in a place that had been silent for generations might precipitate a further collapse of roof or wall.

We crept on again, but after a few more steps I stopped. My torch was picking out some difference in the texture of the floor ahead. Instead of light grey, it looked black. I stared for a minute, then said, ‘Shit! It’s water. The fucker’s flooded.’

‘Never,’ said Pavarotti. ‘If part of the tunnel was flooded, the whole thing would be full of water.’

I saw the logic of what he said – but he was wrong. At the point where the water started the floor was still dropping away, so that as we continued forward the flood gradually deepened. The water was cold and black and stank of decay, and we had no option but to wade into it until we were knee, then thigh, then bollock deep. Only when the surface was above our waists could we see that, a few more yards ahead, it came right to the roof.

‘Jesus Christ!’ said Pavarotti. ‘We’re knackered. We can’t get through this lot.’

We pulled back and started wringing the filthy, black water out of our trousers.

‘Pretty obvious, isn’t it?’ said Pav. ‘Of course it’s going to be flooded, under the bloody river.’

For a minute I sat on the deck, holding my head in my hands, trying to think constructively.

‘There’s no way we’re going to get closer to the Kremlin anywhere else.’

‘Why not forget this bastard?’ Pav suggested. ‘Get the other one in first and then see?’