What dark smooth plain on the surface of the moon?

The dark smooth plains on the surface of the Moon are known as "maria" (singular: "mare"), which are large basaltic plains formed by ancient volcanic eruptions. These areas appear darker than the surrounding highlands due to their composition and smoother texture. The most well-known maria include the Sea of Tranquility (Mare Tranquillitatis) and the Sea of Serenity (Mare Serenitatis). The term "maria" is Latin for "seas," reflecting the early belief that these dark areas were actual bodies of water.