A large iron-rich basalt plain on the lunar surface that was formed by cooling lava. They are generally much smoother and darker than surrounding regions such as the highlands. They are known as "seas" or more commonly known by their Latin name, "mare", because early astronomers believed they were bodies of water.
The Apollo 11 mission that carried Neil Armstrong and Buzz Aldrin to the Moon in July 1969 landed in the Sea of Tranquility.