In the OT levels, Hubbard explains how to reverse the effects of past-life trauma patterns that supposedly extend millions of years into the past. Among these advanced teachings is the story of Xenu (sometimes Xemu), introduced as an alien ruler of the "Galactic Confederacy." According to this story, 75 million years ago Xenu brought billions of people to Earth in spacecraft resembling Douglas DC-8 airliners, stacked them around volcanoes and detonated hydrogen bombs in the volcanoes. The thetans then clustered together, stuck to the bodies of the living, and continue to do this today. Scientologists at advanced levels place considerable emphasis on isolating thetans and neutralizing their ill effects . . .