 Mare Imbrium is not as flat as it was originally because later events have altered its surface.Mare Imbrium may have been formed when a proto-planet from the asteroid belt collided with the moon 3.8 billion years ago.With a diameter of 1145 km, Mare Imbrium is second only to Oceanus Procellarum in size among the maria, and it is the largest mare associated with an impact basin.