Lake Vadimo

Lake Vadimo (Lat. "Lacus Vadimo") was a small, partially dry, lake of volcanic origin whose waters now are almost fully underground best known as the theatre for the battles between Etruscans and Romans in 310 and 283 BC, in both of which the Romans were victorious.
It is near the ancient Etruscan town of Horta (modern Orte) in the province of Viterbo, in the lower Tiber Valley, not far from Rome. 
The Roman domination made the area a municipality of "Horta" (also "Hortanum").
According to Pliny the Elder, there were floating islands on the lake.