Campus Martius

The Pantheon, extant building of the Campus Martius, Rome, Italy.

Campus Martius, in ancient Rome, a floodplain of the Tiber River, the site of the altar of Mars and the temple of Apollo in the 5th century bce. Originally used primarily as a military exercise ground, it was later drained and, by the 1st century bce, became covered with large public buildings—baths, amphitheatre, theatres, gymnasium, crematorium, and many more temples. The Pantheon is the most notable structure extant. The historian Livy (1st century bce) called the area campus ignifer because of the volcanic smoke often seen there.