“Watch out for that bat behind you,” warned Luca Messina, a civil engineer who moonlights as a speleo-archaeologist with Sotterranei di Roma, Undergrounds of Rome, a local group whose members explore the darkest recesses of the Eternal City.

I ducked as the winged mammal swooped past my ear, then flitted beyond the beam of my headlamp and disappeared in the void ahead. “Don’t worry about those insects,” Messina added, nodding to walls hewn 2,300 years ago, and which I now saw were crawling with spider-like crickets called Dolichopoda. “They aren’t dangerous."