It is the place where the Sybil’s answers to Aeneas’ questions once rang out.
It was first thought to be situated on the banks of Lake Avernus and then was confused with the crypt, but in 1932 Maiuri suggested that the current location of the Sybil’s Cave was the real one, used originally for military purposes , the Cave was subsequently used for religious cult and oracles.
It was also probably connected to the area which lay above the Temple of Apollo.
The corridor (dromos)was 131,50 metres long and was lit by six tunnels dug in the westerns wall in Roman times.
On the eastern side there are three smaller tunnels with steps leading to them, and here the Romans installed three cisterns.
The entrance is trapezoid in shape while the room of the oracles, is formed by three spaces in a Latin cross; the room on the left is thought to be that of the Sybil. This place was also to bury early Christians.