Which mare is the largest on the Moon?

The largest mare-like plain on the Moon is Oceanus Procellarum, the “Ocean of Storms.”

Oceanus Procellarum occupies much of the Moon’s western near side and stretches more than 2,500 kilometers from north to south. It covers roughly 4 million square kilometers, about 10.5% of the Moon’s total surface area. Its exceptional size is why it is called an “oceanus” rather than the more usual “mare,” the Latin word for sea.

The dark surface is mostly basalt produced by ancient volcanic eruptions. Although the region looks like a single enormous impact basin, scientists continue to debate its origin. Proposed explanations include a giant impact, large-scale internal heating, and tectonic structures revealed by lunar gravity measurements.

Oceanus Procellarum is sometimes confused with Mare Imbrium, a large neighboring plain to the northeast. Several smaller maria and bays lie around its edges, and the region has hosted numerous robotic landings, including Surveyor 1, Luna 9, and China’s Chang’e 5 in 2020.