Caloris Planitia is a plain within a large impact basin on Mercury, informally named Caloris, about in diameter. It is one of the largest impact basins in the Solar System. Calor 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 the Caloris Montes, a ring of mountains approximately tall.