The cave , 54 m ( 177 ft ) long , 15 m ( 49 ft ) high , and 30 m ( 98 ft ) wide , is believed ( because of the man-made niches ) to have been a nymphaeum , a kind of watery boudoir for the Emperor Tiberius , who retired to Capri in the first century a.d. and built a villa directly above .
The cave is 30 m wide and 15 m high .