Oceanus Procellarum, due to its size: Oceanus Procellarum is the largest of the maria ("seas"), stretching more than 2,500 km (1,600 mi) across its north–south axis and covering roughly 4,000,000 km² (1,500,000 sq mi), accounting for 10.5% of the total lunar surface area.

Where is the Oceanus Procellarum?

The Ocean of Storms (Oceanus Procellarum) on the moon is a vast volvanic plain on the western edge of the moon's near side.

When was Oceanus Procellarum created?

It formed long ago, about 3.5 billion years ago, and subsequent impacts (some of which have been huge) have distorted the outer boundaries. Because of its huge size and assumed roundness, its origin has generally been attributed to something very large hitting the Moon, punching through the crust.