Oceanus Procellarum

The Ocean of Storms sprawls 2,592 km across the western near side, covering roughly four million square kilometres — more than any other mare, and large enough to be obvious to the naked eye as the dark western half of the Moon.

Unlike Imbrium it sits in no clean basin. Its irregular outline and the absence of a defining ring have kept its origin contested: an ancient, heavily degraded mega-basin, or a region where a hot interior simply produced more volcanism than elsewhere. GRAIL gravity data revealed a rectangular pattern of rifts beneath it, which argues for the second explanation.

Surveyor 1, Luna 9 and Apollo 12 all landed here.