The party twisted and turned, this way and that, down tunnels and what once would have been flights of steps; swimming against a slight current. All were focussed on the orange tape before them, when Andy stopped and gestured upwards. They broke through a shallow air pocket. The pocket had been built up from air exhaled in previous dives, which had got trapped in the arched roof. Andy lifted his mask, spat out his respirator and shone his flashlight at the others.

‘Enjoying it? All well?’ Some strangled grunts spluttered back.

‘Okay – we’re making good progress. Two minutes ahead of time. This is where we’ll decompress on our return. We now make a right to the platform and then head into the tunnel. Any questions?’ He waited a few moments. ‘Good. Let’s go.’ He looked at his watch. Another fifty-five minutes underwater; an hour max. As they descended further, he could feel the current get stronger and his flashlight beam illuminated four hundred year old graffiti: London is English!

He knew they had reached the platform as the pressure in his ears altered and the sounds changed, becoming perceptibly louder. Each station was haunted with its own low groans, dull thuds and faraway screeches. Andy was a superstitious man. He half believed that they were the spirits of the thousands who had perished down here centuries ago, calling out to each other. He turned around. Feung was just behind him, wisely sticking close. When they arrived at the tunnel entrance, he waited for the others to rally when something flashed into his beam, startling him. Whatever it was, it was a big bugger, he thought.

When all the other divers had mustered around, he gave them the signal to check their air. So far, so good. They were now going to enter the tunnel and fin the best part of three hundred meters to recce a train, which Bill had reported seeing, but which had never yet been entered. As this part of tunnel was unexplored, there would be no cord to follow. Brushing aside a plastic bag which floated into their circle, Andy took the extra cord he was carrying and fastened it to the same piece of metal where the other cord ended. Signalling the others, he then finned headfirst into the tunnel, allowing it to unravel behind him.