There's a technique to squeezing my body through this particular crack in the wall. It involves thrusting myself forward at full stretch, using my left arm to support my body as I wriggle forward to the bend, where my right hand will find a grip to pull myself round the corner. I must take care, however, not to slip to the bottom, where I'll be wedged in to join the unfortunate club of 'stuckies'. Dust is glittering in the glow of my headlamp and I can hear the others crawling ahead. As I shuffle into position, I wonder what's happening above my head on the streets of Budapest.

Millions of years ago, a sea flowed beneath what would become the Hungarian capital, creating a vast network of caves and underground wells that today feed its famous thermal baths. In Buda (one side of the city) it's possible to explore these caves on excursions marketed primarily to backpackers and companies for team-building exercises.

The Matyas Caves are about 20 minutes by bus from downtown and closed to the public. Used as bomb shelters during the second world war, the caves extend for many kilometres, although our three-hour adventure covers a circle of only about 800 metres. The guide, Kata, unlocks a thick iron door, issues helmets and lamps, and ushers us in. When the door is closed, the darkness and silence are immediate. We descend 10 metres down an old iron ladder and the challenge has begun.