This is a recent archeological discovery, since it was thought to have been totally destroyed by Trajan, 30 years after Nero’s death. Actually, it was partially destroyed and buried by the Trajan Baths, but they found remains of the previous building and they are still excavating. Because of that, it is only open to the public on Fridays, Saturdays and Sundays (excavations continue during the rest of the week), and you can only visit on guided group tours (check when there are English language tours and reserve weeks in advance). The tour is quite impressive, includes a virtual reality part with VR goggles, and lasts about 90 minutes.