Thomas Edison is credited with trying to invent a machine designed to contact the dead.

It never quite worked, but it laid the groundwork for a belief in the relationship between electronics and spirits. Today, the use of "electronic voice phenomena," or EVPs, is one of the most highly contested and popular methods for paranormal investigators.

Investigators said voices left behind on digital recorders prove the existence of ghosts.

Tom McDonough, an astrophysicist and senior scientist at the Skeptic Society in California, encourages all investigations -- including the paranormal. He just thinks people need to be careful.

McDonough argues that the mind can play tricks.

"Like people who think they see an image of the Virgin Mary on a piece of toast," he said, "groups of people who want to hear something very well may."