The belief system of Scientology features the myth of the alien overlord Xenu, who solved an overpopulation problem in the worlds of his Galactic Federation by removing the "thetans," the soul or life force, and putting them in volcanoes on Earth. This caused Xenu's federation to revolt against him.
Scientology founder L. Ron Hubbard taught that Xenu and his extraterrestrial federation existed about 75 million years ago in an unspecified portion of the universe. Thetans, Hubbard claimed, are not just the soul, but the only truly living portion of a sentient being. All sentient beings have a thetan attached, and the thetan is not only immortal, but has always existed. Thetans are reputed to have created the universe just by agreeing that it exists. Understanding the inner thetan is the core of Scientology, and adherents are charged hundreds of thousands of dollars by the church to get in touch with the thetan and unlock its special abilities.
Other than the thetans that already exist here, aliens are prohibited from visiting Earth and the solar system. When Xenu removed the thetans, his federation misunderstood and thought that he was murdering billions of people. Therefore, after the successful revolt to depose him, he was imprisoned on Earth and the entire area proscribed. While, Hubbard explained, there have been alien visitors to Earth since these events, these visits were illicit and probably criminal in the eyes of the Galactic Federation.