Sea of Tranquility

The 'sea' where humans took their first step

The Moon's 'seas' have no water! They are plains of ancient lava that flowed out and hardened — that's why they are dark and flat. The Sea of Tranquility is the most famous: in 1969 Apollo 11 landed right here, and Neil Armstrong said 'The Eagle has landed!' Look at the Moon: the big dark patch at the upper right is this sea.

The Sea of Tranquility (Mare Tranquillitatis) is a basalt plain 873 km across — an ancient impact basin flooded by lava 3.6–3.9 billion years ago. Its basalt is rich in titanium, giving it a slightly bluer tint than other maria. Apollo 11 (1969) and Surveyor 5 (1967) landed here; Ranger 8 (1965) crashed here. The names of lunar seas come from Riccioli's 1651 map — he named them after weather and moods (Tranquility, Rains, Ocean of Storms).

Fun facts

• The maria cover 16% of the Moon's surface — almost all on the near side.
• The rocks Apollo 11 brought back are 3.7 billion years old — older than Earth's oldest rocks.
• Ancient people saw these dark patches as 'the Man in the Moon'.