On July 20th, 1969, humans achieved their first visit to another world when Apollo 11 astronauts Neil Armstrong and Buzz Aldrin landed and walked on the Moon. The lunar location chosen for this historic event, a site of “stark beauty” according to Armstrong and “magnificent desolation” according to Aldrin, was the Sea of Tranquility.

For centuries, the Sea of Tranquility’s relatively dark appearance from Earth had been a source of fascination among astronomers, who believed it to be a lunar ocean. In 1651, it was named accordingly by astronomers Francesco Grimaldi and Giovanni Battista Riccioli: Mare Tranquillitatis, or Sea of Tranquility.

In reality, the Sea of Tranquility’s darker appearance is due to its composition. It is located in the Tranquillitatis basin (centered on 0.68 N, 23.43 E; extending, approximately, from 20.4 N-4.4 S, 15.0-45.9 E), a crater that was likely formed over 3.9 billion years ago by a massive impact. The basin is approximately 540 miles across and overlaps with the surrounding Serenitatis and Nectaris basins. Mare basalts filled the crater, not only giving the region its shaded appearance but also forming a relatively level surface. In fact, the difference between the highest point and the lowest point within the Sea of Tranquility is less than one third of a mile. This comparative flatness, along with its proximity to the equator, made the Sea of Tranquility a prime location for one of the most extraordinary achievements in human history.