Scientology has been discussed in the context of UFO religions in "UFO Religions" by Christopher Partridge, "The Encyclopedic Sourcebook of UFO Religions" by James R. Lewis, and "UFO Religion: Inside Flying Saucer Cults and Culture" by Gregory Reece. Stories of extraterrestrial civilizations and interventions in past lives form a part of the belief system of Scientology. The most well-known story publicized and held up to ridicule by critics is that of Xenu, the ruler of the Galactic Confederacy who is said to have brought billions of frozen people to Earth 75 million years ago and placed them near a number of volcanoes, and dropped hydrogen bombs into them, thus killing the entire population in an effort to solve overpopulation. The spirits of these people were then captured by Xenu and mass implanted with numerous suggestions and then "packaged" into clusters of spirits.