So what are ghosts? They are believed to be dead people who haven't yet passed over to the ?other side?. They are stuck in limbo for one reason or another, trapped in our physical world. Some people say this is because of a traumatic death, or because of tragedy.

It is believed also that ghosts ?haunt? sites that they enjoyed in life, or the place of their death. So if we know where a particular ghost is likely to show up, why is there not photographic evidence? Of course there are lots of photographs in circulation of what appear to be ghosts. The trouble is that experts can tell if a photo has been tampered with, but they can't say for sure if the subject matter in the photo is a ghost or not.

Many of the ghost photographs in circulation are images of ghostly orbs. An orb is believed to be a ghost in the form of a ball of light. Experts in the ghost-hunting field explain that orbs are less shy and allow people to photograph them. Of course sceptics say that orb type ?ghosts? are easier to falsify in a photo.

However, ghosts do a lot more than just putting in the odd appearance. Report have been made of ghosts banging, rapping, making footsteps, moving things around, hiding objects and returning them at a later date, slamming doors, flicking light switches and generally behaving like mischievous ghosts.

The sceptics would say that all such activities could have a rational explanation, and that ?ghostly? behaviour was typical ghostly haunting of any classic ghost tale, which obviously says more the human brain and the power of projection than it says about ghosts.

However, scientists are still studding the paranormal, which means that ghosts are not yet completely ruled out. What do you believe? What proof would you need to believe in ghosts?