The origin story of Scientology (an organization some call a religion, and others call a cult) begins with an evil alien dictator named Xenu. Scientologists believe that Xenu killed a huge alien population on Earth with volcanoes and hydrogen bombs 75 billion years ago. Then, the alien souls rose up from Earth and entered human bodies, implanting in them false memories and images. These alien souls are called "thetans." Scientologists believe thetans are the cause of mental and physical health issues, and that those problems can be solved with spiritual intervention.