In the world of spirits and apparitions, the poltergeist is a unique type of ghost -- one known for its alleged ability to move physical objects (often in violent, unpredictable ways), to target and torment specific human beings and, perhaps strangest of all, to suddenly, one day, disappear. While not everyone uses the name 'poltergeist,' legends of these and similar creatures occur in numerous civilizations throughout history. Some stories are proven hoaxes. But others, the true believers argue, are more difficult to explain.