Mare Imbrium (Latin for "Sea of Showers" or "Sea of Rains") is a vast lava plain filling the Imbrium Basin on the Moon. One of the larger craters in the Solar System, Mare Imbrium formed when lava flooded the giant crater formed when a very large object hit the Moon long ago. Its age has been estimated using U-Pb dating methods to 3938 ± 4 million years ago. 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.