Short answers, pulled from the story.
Mare Tranquillitatis, Latin for Sea of Tranquillity, is a lunar mare sitting within the Tranquillitatis basin on the Moon. It is composed of basalt from the Upper Imbrian epoch, and its basin is probably Pre-Nectarian in origin. It is notable as the site of the first crewed lunar landing.
Mare Tranquillitatis was named in 1651 by astronomers Francesco Grimaldi and Giovanni Battista Riccioli in their lunar map Almagestum novum. Before that, Michael van Langren had called the same feature Mare Belgicum in his 1645 work Lumina Austriaca Philippica.
Apollo 11 landed in Mare Tranquillitatis on the 20th of July 1969 at 20:18 UTC. Astronauts Neil Armstrong and Buzz Aldrin made a soft landing in the Lunar Module Eagle, and the landing area was later designated Statio Tranquillitatis.
Ranger 8 was deliberately crashed into Mare Tranquillitatis on the 20th of February 1965 after transmitting 7,137 photographs in its final 23 minutes. Surveyor 5 then landed in the mare on the 11th of September 1967, having transmitted 19,118 images of the Moon.
Mare Tranquillitatis has a slight bluish tint compared to the rest of the Moon, which becomes visible when color is processed and extracted from multiple photographs. The color is likely caused by a higher metal content in the basaltic soil or rocks of the mare.
Unlike many other maria, Mare Tranquillitatis has no mass concentration, or mascon, at its center. Mascons were identified in other maria such as Serenitatis and Imbrium from Doppler tracking of five Lunar Orbiter spacecraft in 1968; later missions including Lunar Prospector and GRAIL revealed an irregular gravity pattern over Tranquillitatis instead.