They kept moving at the same trotting pace. The mouth of the tunnel was a long way behind them now, just a white semicircle that vanished out of sight as they followed the curve of the parallel tracks. Moments after, Ben sensed the tunnel beginning to vibrate, a subtle tremor at first, then quickly building in intensity as the tracks started to thrum and the rumble of the approaching train grew louder. It paused to pick up the passengers from the platform, then accelerated into the tunnel. The rumble became a loud, breathy roar. First Jafari, then Roth, then Ben, made it to the next archway, and they hid behind the crumbly stonework as the train streaked by with a slap of wind and a deafening screech, its windows a blur.

Then the train was gone again, wending its way into darkness beneath the city. The three of them emerged from their hiding place and pressed on. Jafari motioned them towards the parallel tunnel to the left. By the glow of the wall lights it appeared older and dirtier and less used. He explained that this was part of the disused network. ‘But be careful as we cross the tracks. The live rail is still connected to the grid and it’ll fry you like an egg.’

‘Thanks for the thought, Hasan,’ Ben said.

Jafari shrugged. ‘If I let one of you step on a live rail, I figure the other will shoot me in the legs, then take me back to that stinking fucked-up shithole place and turn the dog on me again. So I do it only for myself.’

‘You’re a man of remarkable perception. Now keep moving.’

They crossed the disused line to the walkway on the far side and walked on along the abandoned tunnel, which gently curved away until the other disappeared out of sight behind them. The glow of the wall lights was dulled by grime. It felt like a thousand miles below the earth, deep in the bowels of the labyrinth from which it was easy to imagine there could be no escape.

Roth whispered, ‘How much further?’

Jafari replied over his shoulder, ‘We’re getting closer.’

After a hundred more yards, they came to a heavily graffiti-scrawled iron door on the left, fitted with a panic bar handle that had been recently used, hand marks visible against the dirt. Jafari said, ‘Through here.’

Jafari pushed the door open and stepped through. Roth went next, Ben last. They had emerged into the abandoned station of Saint-Martin.