Mare Tranquillitatis (Latin for Sea of Tranquility) is a lunar mare that sits within the Tranquillitatis basin on Earth's moon. The mare material within the basin consists of basalt in the intermediate to young age group of the Upper Imbrian epoch. The surrounding mountains are thought to be of the Lower Imbrian epoch, but the actual basin is probably Pre-Nectarian. The basin has irregular margins and lacks a defined multiple-ringed structure. The irregular topography in and near this basin results from the intersection of the Tranquillitatis, Nectaris, Crisium, Fecunditatis, and Serenitatis basins with two throughgoing rings of the Procellarum basin. Palus Somni, on the northeastern rim of the mare, is filled with the basalt that spilled over from Tranquillitatis.
In 1965, the Ranger 8 spacecraft crashed in Mare Tranquillitatis, after successfully transmitting 7,137 photographs of the moon in the final 23 minutes of its mission. This mare also served as the landing site for the Apollo 11 lunar module, the first manned landing on the Moon. The landing area at 0.8° N, 23.5° E has been designated Statio Tranquillitatis, and three small craters to the north of the base have been named Aldrin, Collins, and Armstrong in honor of the Apollo 11 astronauts.
This Mare has a slight bluish tint relative to the rest of the moon and stands out quite well when color is processed and extracted from multiple photographs. The color is likely due to higher metal content in the basaltic soil or rocks.
Sea of Tranquillity is the title of a 1994 novel by Paul Russell.
"Sea of Tranquility" is also a song by the progressive rock band Barclay James Harvest, included in their 1977 album Gone to Earth. The composition was written by the band keyboardist Stuart "Woolly" Wolstenholme.
Howard Korder wrote a play called Sea of Tranquility.
The Brazilian rock band from the 80's RPM mentions the Sea of Tranquility in their hit song "Radio Pirata".