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The Sea of Tranquility was a location on the Moon. In 2049, the Twelfth Doctor discovered lines of tectonic stress coming out of the Sea of Tranquility and other locations all over the Moon. These cracks turned out to be a giant creature beginning to hatch from inside the Moon after growing for a hundred million years. (TV: Kill the Moon)
According to another account, the Moon originally orbited 16 Alpha Leonis One. The Cthalctose used it as an ark to save their species. They launched it into space around 6 billion BC, and it eventually became the Earth's moon. Their terraforming platform was built in what later would be known as the Sea of Tranquility, and incorporated into Tranquillity Base. (PROSE: Eternity Weeps)