Among the legends of ancient Rome is that of a sibyl who lived in a cave that was the portal to the underworld. In 1958, a cave was discovered on the north shore of the Bay of Naples among the ruins of Baiæ that may have given rise to this legend. It opens to a passageway that is heated by an active volcano. Robert Paget and Keith Jones spent years removing rubble, exploring the cave's layout, mapping, and speculating on its purpose.
But only when the men went deeper into the hillside did the greatest mystery of the tunnels revealed itself. There, hidden at the bottom of a much steeper passage, and behind a second S-bend that prevented anyone approaching from seeing it until the final moment, ran an underground stream. A small “landing stage” projected out into the sulfurous waters, which ran from left to right across the tunnel and disappeared into the darkness. And the river itself was hot to the touch–in places it approached boiling point.
Conditions at this low point in the tunnel complex certainly were stygian. The temperature had risen to 120 degrees Fahrenheit; the air stank of sulfur. It was a relief to force a way across the stream and up a steep ascending passage on the other side, which eventually opened into an antechamber, oriented this time to the helical sunset, that Paget dubbed the “hidden sanctuary.” From there, more hidden staircases ascended to the surface to emerge behind the ruins of water tanks that had fed the spas at the ancient temple complex.