High Quality Content by WIKIPEDIA articles! The Caloris Basin, also called Caloris Planitia, is a large impact crater on Mercury about 1,550 km (960 mi) in diameter, one of the largest impact basins in the solar system. Caloris is Latin for heat and the basin is so-named because the Sun is almost directly overhead every second time Mercury passes perihelion. The crater, discovered in 1974, is surrounded by a ring of mountains approximately 2 km (1.2 mi) tall.