This video provides part of a documentary about the early 1970s experiment in which a group of people attempted to re-create the effects of a seance by inventing an imaginary ghost. And it worked, resulting in psychokinetic and other unexplained phenomena. You can read all about the experiment here
This is one of the most fascinating -- and possibly one of the most important -- experiments in parapsychology. Its effect could provide insight into the nature of poltergeist and haunting activity. Actual footage from the experiment is included.