When is a grotto not "grotesque"? When it's the famous blue grotto of Capri, where one
of the late Roman emperors had a hideaway, can't remember his name.
"The Blue Grotto is accessible only by sea and is known for
its incredible iridescent blue water and reflections on the
cave walls caused by refracted sunlight penetrating
through a large opening beneath the water. You may be
taken in by independently operated boats or swim in.
However, swimming in the Blue Grotto is only allowed
before or after the independently operated boat tours'
hours of operation.

Edit: I found the Emperor's name. Tiberius. Here is a advertisement that does have a lot
of descriptive material"
http://www.hotelolimpico.it/downcapri.htm