In a little-frequented area of the park, a moss-lined lane overhung with trees leads downwards to a metal door. Once opened by the guides, it leads into the huge quarry complex.

For centuries, legions of slaves extracted huge quantities of pozzolana, a volcanic rock which they pulverised to create a type of ancient concrete. Mixed with lime, it was used in the construction of vast edifices such as the Pantheon temple and the Baths of Caracalla, a huge thermal complex not far from the Circus Maximus chariot racing track.

“The reason the Romans dug this underground quarry so close to the city was because transportation was so expensive. Building materials are heavy and if you want to bring them from far away, you have to pay the workers, you need to feed the beasts of burden, there is the danger of brigands. Much cheaper to excavate right here,” said Alessandro Placidi, a guide from Sotterranei di Roma or Underground Rome, a group of speleologists and historians who specialise in exploring the city’s hidden subterranean places.

“The real Rome is hidden underground, a precious treasure that few people know about or appreciate,” the group says on its website.

At one point he asks the group of around 40 cyclists to turn off all bike lights and mobile phones. The darkness is absolute – not a single shape or shadow can be made out. The thought of being lost in this labyrinth without a source of light is terrifying.