 Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today.The basin's age has been estimated using uranium–lead 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.