I’d dropped twelve metres under the city streets – if not out of mind, then at least, for the moment, out of sight. But the city wasn’t simply hiding me now, it had buried me: no satellites, no thermal imaging, no cell phone signal – and no CCTV. It was as if modern technology had never happened. Above all, my one advantage persisted: they didn’t know where I was going, or why. The fact that I didn’t either was, I tried to convince myself, neither here nor there. All I had to do was stay the one step ahead that I’d kept up since diving into the Channel, and hope that I might find some answers before I was tripped up for good.

I turned my back on the crook of the passage. So many of the tunnels around the Place d’Italie were blocked off that travelling in a straight line was impossible. It had been years since I’d been down there, too – and although the tunnels had been hewn out over two centuries before, they’d never stayed the same for long. An organic, ever-changing network, it was more like the intestines of the city than its foundations. Ultimately, I needed to be going south-west, but to take that bearing I’d first have to head south-east and then loop round.

I set off into the world’s largest catacombs still guided by my father’s helping hand. But the sheer immensity of the labyrinth was almost overwhelming. The rough earth floor was strewn with small loose stones; the walls were mined rock, patched with brick and injections of cement. I walked slowly, head down. After twenty metres a short passage opened up on the left, which terminated in a T-junction after only a few paces. On the wall to the right of the entrance, another cross, with another marker.

Keep going, it said.

Ten metres more and the tunnel made the dog-leg I’d seen from the foot of the ladder. The passage sloped downwards; the earth underfoot gave way to mud; the ceiling height fell rapidly. I stopped and listened and went dark again. There was a decision to make. Keep the torch on and move faster – but risk being seen. Or go dark and rely on only the M4’s laser sight – that would allow me to see basic changes in the tunnel terrain, like a blind man sweeping a stick in front of him, but it would slow my progress to a crawl.

Slow and steady wins the race.