Mare Imbrium (Moon)

"Sea of Showers"

Mare Imbrium is a vast lunar mare filling a basin. The maria have fewer features than other areas because molten lava pooled in the craters and formed a smooth surface. Mare Imbrium is not as flat as it was originally because later events have altered its surface. (wikipedia entry)

A mare is a "sea" or large circular plain. On the Moon they are named after latin terms describing weather and other abstract concepts.