Mare Imbrium (Latin for "Sea of Showers" or "Sea of Rains") is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System, if even that. The Imbrium Basin formed from the collision of a proto-planet during the Late Heavy Bombardment. Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today. The basin's age has been estimated using U-Pb dating methods to 3938 ± 4 million years ago, the diameter of the impactor has been estimated to be 250 ± 25 km. The Moon's maria (plural of mare) have fewer features than other areas of the Moon because molten lava pooled in the craters and formed a relatively smooth surface. Mare Imbrium is not as flat as it was originally because later events have altered its surface.