The Phlegraen Fields and the Tunnels of Baiae

Wonderful little article at Smithsonian on the strange tunnel complex at Baiae, on the volcanic northern shore of the Bay of Naples. This area was known in antiquity as the Campi Flegrei, the Fields Devoured by Fire. A relatively quiet period in its geological history corresponded to the glory days of Rome, and the area was developed as a resort with many different sulfurous baths. Since then the underlying volcano has flared up several times, and much of the Roman resort is now under water.

Much survives, though, including a temple complex within which, it was long known, was the opening of a tunnel or antrum running into the hillside:

The antrum at Baiæ proved difficult to explore. A sliver of tunnel, obviously ancient and manmade, disappeared into a hillside close to the ruins of a temple. The first curious onlookers who pressed their heads into its cramped entrance beat a hurried retreat–the pitch-black passageway was uncomfortably hot and wreathed in sulfurous smoke. There the mystery rested while the Second World War intervened, and it was not revived until, early in the 1950s, the site came to the attention of Robin Paget. Paget was not a professional archaeologist. He was a Briton who worked at a nearby NATO airbase, lived in Baiæ, and excavated mostly as a hobby. 

Paget and his friends eventually cleared and mapped the tunnel, and they found that it led to a complex of chambers deep under ground. Within one chamber is an underground stream so hot that it sometimes boils. Theories about the chambers abound: they were the home of the Sybil of Cumae, or they represent a sort of model of the underworld through which initiates would be led, the boiling stream standing in for the River Styx. Whatever it was, it's a very cool site.