Mare Tranquillitatis, or the Sea of Tranquility, is a large, flat area on the surface of the Moon. It is a basaltic plain formed by ancient volcanic activity and is one of the most well-known lunar maria. The region is characterized by its smooth, dark surface and is located in the Moon's northeastern hemisphere. This area gained historical significance as the landing site of the Apollo 11 mission in 1969, where astronauts Neil Armstrong and Buzz Aldrin became the first humans to walk on the Moon. Mare Tranquillitatis is often studied for its geological features and is a popular subject in lunar exploration.