The lake is known as the Mouth of Hell. The volcano to the east arose out of nothing in the 1300s (?) and destroyed the remains of a roman city. This area was quite the summer resort in Roman days.
The dome here is the remains of the "Temple of Apollo" from the first century. It is the second largest dome built by the Romans after the Pantheon in Rome.
http://en.wikipedia.org/wiki/List_of_Roman_domes#Domes