Beneath Toledo's old city runs a network of Roman cisterns and tunnels, the most famous of which is the Cave of Hercules — a vaulted underground space dating to Roman times, located in the alley of San Ginés. For medieval Toledans, it was a place of mystery and legend; Arab geographers mentioned it, Christian chroniclers elaborated the stories, and the cave accumulated a mythology far larger than its modest dimensions.
Today visitors can descend into the Roman cisterns and see the original construction — brick vaulting, niches in the walls, the engineering of a city that needed to store water on a rocky hilltop with limited springs. It is genuinely atmospheric: cool, dimly lit, smelling of old stone. The cave is accessed through a small entrance near the Cathedral.
Tours run in Spanish and sometimes in English — check opening times before you go. The visit takes about 45 minutes and combines well with the Cathedral and the Mosque of las Tornerías, both nearby.