Oceanus Procellarum, Latin for "Ocean of Storms", is a vast lunar mare on the western edge of the near side of Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather. Procellarum is the largest of the lunar maria, stretching 2500 kilometers across its north-south axis and covering roughly 4,000,000 km2.
Like all lunar maria, Oceanus Procellarum was formed by ancient basaltic flood volcanic eruptions that covered the region in a thick, nearly flat layer of solidified magma. Unlike the other lunar maria, however, Procellarum is not contained within an impact basin. Around its edges lie many minor bays and seas, including Mare Nubium and Mare Humorum to the south. To the northeast, Oceanus Procellarum is separated from Mare Imbrium by the Carpathian Mountains.