Mare Smythii is a lunar mare located along the equator on the easternmost edge of the lunar near side. The Smythii basin where the mare is located is of the Pre-Nectarian epoch, while the surrounding features are of the Nectarian system. The mare material, which make up the floor of the mare, is a high aluminous basalt, and consists of Upper Imbrian basalt covered by Eratosthenian basalt. Mare Smythii contains a high number of craters with fractured floors, including the noticeably unusual crater Haldane. (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.