One theory which might sound unbelievable is that the Moai were created and placed on the Easter island by aliens. This insane sounding idea was put forth in the book- Chariots of the Gods?: Unsolved Mysteries of the Past by Erich von Daniken. Not only the Moai, von Daniken thinks that even the Pyramids in Egypt are works by the extraterrestrial.
The argument to this is- the stone used to build the Moai were from the island itself.