The tunnel, with a length of about 780 m (width: 4-7 m; height: 4-9 m), crosses the tufa hill of Posillipo, connecting the area of Bagnoli and Campi Flegrei with the Vallone della Gaiola.
Of the three tunnels, which gave light and ventilation to the route, only two remain open (in one of them Sir Arthur Conan Doyle, the creator of Sherlock Holmes, got stuck during a visit).
Past the cave, a path lined with typical Mediterranean vegetation leads to the area of Pollione's villa (which, before the tunnel, could only be reached by sea).
The area includes extraordinary archaeological remains of a theater with a splendid structure, built exploiting the natural slope of the hill (according to the typical Greek technique): 13 orders of seats in the ima cavea and with 6 in the middle one for a total capacity of two thousand seats.
A garden divides the stage of the major theater from a second theater: the Odeion (gr. oidêion, from oidé, "singing"), smaller in size, covered and with a smaller cavea. It was intended for rhetorical poetry and music auditions.
Inside there are some rooms with mosaic and marble floors and remains of painted wall coverings.The other ruins that have emerged so far are: to the east of the Great Theater, the Temple or Sacrarium; to the west, the Nymphaeum with traces of a bath facility. It is currently possible to visit a stretch of about 2 km, from the Coroglio Descent, through the Sejan Cave, to the area of the theaters.
