meteor crater?
Page 262/263 Worlds In Collison:
'Bolsena, or the ancient Volsinium, was one of the chief cities of the Etruscans, the people whose civilization preceded that of the Latin Romans on the Apennine Peninsula. The Estruscan states occupied the area of what was later known as Tuscany, between the Tiber and the Arno.
'Near Bolsena, or Volsinium, is a lake of the same name. This lake fills a basin nine miles long, seven miles wide and 285 feet deep. For a long time this basin was regarded as the water-filled crater of a volcano. However, its area of 117 square kilometres exceeds by far that of the largest known craters on the earth - (snip)
'Hence, the idea that the lake is the crater of an extinct volcano has recently been questioned. Moreover, although the bottom of the lake is of lava, and the ground around the lake abounds with ashes and lava columns of basalt, the talus of a volcano is lacking...'