It was not a journey to be undertaken without maps – though maps alone, my father had impressed upon me, were not enough to save anyone down there. Most of the tunnels were hardly tall enough to stand up in; some shrank to barely a foot high. Seemingly bottomless wells that plumbed the black abyss of underground aquifers; cave-ins; flooded tunnels; eye-gouging metal spikes; ankle-breaking holes; and deep, dark pits that could swallow a man all lay in wait. The fresh corpses of careless explorers were easily added to the ancient, unnamed remains.

All those years ago we’d emerged from that vast, random city of the dead in Montrouge Cemetery, south of the centre near the Périphérique – the busy highway that ringed Paris. If I could escape the city, I could escape – period.

The stretch of tunnel where I stood formed the elbow of a bend. Right led south-east; left, north-west – where, barely two metres further on, the continuation of the tunnel was also masked by a crook in the stone passage. I straightened carefully from my crouch, turned about face – and looked up. The footholds I’d used to drop down the shaft disappeared into the blackness above. I killed the torch, and kept staring up, listening, squinting into the darkness.

Still nothing.

If anyone had followed me underground, they’d used a different entrance. I switched the torch on again and played the beam across the rock around the crude ladder. The name of the street above was chiselled into the stone in mid-nineteenth-century letters, along with the date the passage was reinforced: 1865. But in between these signposted convergences, the routes of the tunnels and the city streets diverged wildly. I looked carefully around the inscription, putting my hand on the cool stone as if that would guide my eyes. I found what I was looking for almost immediately: two small sets of letters carved at head height.

MMG JMG

Max and John Mac Ghill’ean. The initials were scratched deeper than I remembered; and although covered with fine green moss-like mould, they were as clear as the night we’d cut them, if you knew where to look. I traced my fingers over my father’s initials and slipped into a kind of shock. It was the only physical trace of him that remained. Beneath the letters we’d carved a St Patrick’s Saltire. To the left of where the two arms of the cross joined, a dot, bored into the stone with the punch of a Swiss Army knife.

Turn left.

And in the V that made the top of the cross, another.

Climb up.