The term 'SEA of Tranquility' is a reference to the SEA of Tranquility, a large, dark, basaltic plain on the Moon. It was the site of the first human landing on the Moon, when Apollo 11 astronauts Neil Armstrong and Edwin 'BUZZ' Aldrin became the first humans to set foot on the lunar surface in 1969. The SEA of Tranquility is located in the northern hemisphere of the Moon and is one of the most prominent features of the lunar surface.