The mystery of the 'man in the moon' has been solved by scientists, who say it was a volcano that caused the giant basin, not an asteroid impact as previously thought.
MIT scientists have used Nasa data to examine the Procellarum, an area on the near side of the moon which is a giant basin almost as wide as the US, commonly known as the man on the moon.
Previously, scientists have suggested the basin was formed by a massive asteroid impact.
However, researchers have now created a high-resolution map to find that its border is composed of sharp angles that could not have been created by an impact - asteroids normally produce circular craters.
Instead, the scientists believe it was produced by giant tension cracks in the moon's crust following an upwelling of a large plume of magma from deep within the interior.
Published in the journal Nature, Maria Zuber, vice president for research at MIT, said that the cracks formed a "plumbing system" that allowed magma to the surface and eventually filled the area's smaller basin.
Researchers discovered a series of linear gravitational anomalies running beneath the basin, which appear to be remnants of ancient rifts in the moon's crust. It was these rifts, the authors say, that provided a vast magma plumbing system that flooded the area with lava between three and four million years ago.
They said the steep difference in temperature between the magma and the surrounding crust caused the surface to contract, creating the pattern of fractures that allowed for molten material to rise up.
Study co-author Jim Head, from Brown University, said: "We think this is a really good, testable alternative to the impact basin theory. Everything we see suggests that internal forces were critical in the formation of Procellarum."
Zuber added: "How such a plume arose remains a mystery. It could be due to radioactive decay of heat-producing elements in the deep interior. Or, conceivably, a very early large impact triggered the plume.
"But in the latter case, all evidence for such an impact has been completely erased. People who thought that all this volcanism was related to a gigantic impact need to go back and think some more about that."