Campus Martius, a plain in ancient Rome. It covered an area of about 600 acres (240 hectares) between the Quirinal, Capitoline, and Pincian hills and the Tiber River. The name means “Plain of Mars,” the field being sacred to the god Mars. The Campus Martius was used for military exercises, athletic contests, and public law-making assemblies. Beginning in the first century B.C., public and private buildings—including the Pantheon—were built on it. A business district of modern Rome now occupies the site.