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 through going rings of the Procellarum basin. Palus Somni, on the northeastern rim of the mare, is filled with the basalt that spilled over from Tranquillitatis.
This Mare has a slight bluish tint relative to the rest of the moon and stands out quite well when colour is processed and extracted from multiple photographs. The colour is likely due to higher metal content in the basaltic soil or rocks.
Unlike many other maria, there is no mass concentration (mascon), or gravitational high, in the center of Mare Tranquillitatis. Mascons were identified in the center of other maria (such as Serenitatis or Imbrium) from Doppler tracking of the five Lunar Orbiter spacecraft in 1968. The gravity field was mapped at higher resolution with later orbiters such as Lunar Prospector and GRAIL, which unveiled an irregular pattern.
This mare was the landing site for the Apollo 11 mission, the first crewed landing on the Moon on July 20, 1969, at 20:18 UTC. After astronauts Neil Armstrong and Buzz Aldrin made a smooth touchdown in the Apollo 11 Lunar Module, named Eagle. Armstrong told flight controllers on Earth, "Houston, Tranquility Base here. The Eagle has landed." The landing area at 0.8° N, 23.5° E has been designated Statio Tranquillitatis after Armstrong's name for it, and three small craters to the north of the base have been named Aldrin, Collins, and Armstrong in honor of the Apollo 11 crew. The Apollo 11 landed at 00.67408° North latitude, 23.47297° East longitude.