A mars sized proto-planet named Theia collided with a young Earth. The collision resulted in the formation of the Moon and the Sumpian crater.
Over time, the weathering and erosion caused the Sumpian crater to decrease in size.
The size of the crater was documented when an early Perbian civilization settled on Earth about 1.5 billion years ago as "...fifty [meters] in diameter, and twenty [meters] in depth."
This civilization thrived for millions of years, and soon, the Perbs began forming bonds with Earth-born microorganisms.
Some Perbs began to go to other planets such as Venus and Mars from Earth, which attracted the attention of Gleengi.
The migration of Gleengi to Earth that would ensue is known today as the Great Gleengian Migration