In which lunar mare did Apollo 11 land?

Apollo 11 landed in Mare Tranquillitatis, the lunar mare commonly called the Sea of Tranquility. The Lunar Module Eagle touched down there on July 20, 1969, establishing what became known as Tranquility Base.

A lunar mare is not an ocean of liquid water. The dark plains formed when ancient volcanic eruptions filled large impact basins with basaltic lava. Mare Tranquillitatis is a broad, relatively smooth plain, making it a practical target for the first crewed landing even though the surface still contained craters and scattered rocks.

Neil Armstrong manually guided Eagle away from a hazardous crater and boulder field during the final descent. The actual landing point was close to the planned site but not exactly on the computer-targeted location. Armstrong and Aldrin then spent more than two hours outside the spacecraft, collecting samples and deploying experiments.

Mare Tranquillitatis is distinct from Mare Serenitatis, Oceanus Procellarum, and Mare Imbrium, all of which are other named lunar plains. Its name was given in the seventeenth century by astronomers Francesco Grimaldi and Giovanni Battista Riccioli.