My voice went nowhere, bouncing back at me in a flat, close echo. I checked with my thumb that the fire selector was still on semi-auto, brought the rifle up, pressed my back against stone and went white.

The LED torch beam stretched out from under the barrel of the M4 along thirty metres of low, limestone tunnel – illuminating, at its furthest point, what looked like a dog-leg in the passage. There was just enough clearance for the barrel to traverse the hundred and eighty degrees to light up the other direction. As I whipped the rifle round to my left, the wall directly in front of me flared up in a bright white hotspot. In the middle of it a circle of slack-jawed skulls stared back at me – empty eye sockets filled with deep, moving shadows; teeth missing, broken, discoloured by cave-slime; craniums cracked and mouldy. Around the skulls a ragged circle of femurs set them off like a shield on the passage wall. As I moved the LED beam across them, the shadows made the dead eyes flit this way and that, as if they were following the rifle, examining me.

It was exactly as I remembered. I jumped, anyway.

I was neither in the sewers, nor in the shafts servicing the Métro. I hadn’t ended up in a basement or a siege tunnel. The manhole I’d slipped through wasn’t a door so much as a portal, because it wasn’t just an entrance to an underground passageway; it was a gateway into a parallel Paris.

Beneath the capital’s pavements limestone quarries mined centuries ago to build the houses and churches above spread out in a network of caverns and wells, galleries and caves – all connected by three hundred kilometres of pitch-black passages. But all that, my father had told me as we sank into the darkness together that first time twenty-eight years ago, wasn’t what made Paris unique.

It was what happened after the quarries were dug, he said, that was truly remarkable. The basements and cellars of huge swathes of the city south of the Seine sat on top of – and in many cases, were connected to – more than just an ancient mining complex. In the eighteenth century the abandoned chambers were used as the perfect solution to an unspeakable problem: what to do with the horror show of the capital’s overflowing cemeteries. Into those acres upon acres of suburban caverns had been interred the bones of six million bodies.