Several years ago, I stayed in a small apartment adjoining an old cabin. The property was far from city lights, and on clear nights, the shadows could be a little spooky. Sometimes, especially in the dark evenings of the fall and early winter, I had the uncanny sense that I wasn't alone.

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One night, while I was in the apartment, I heard a muffled thump that seemed to come from inside the cabin. Ordinarily, I would have dismissed the sound as the settling of the century-old building. But it was an eerily still night, and I already found the terrain around the cabin unnerving after dark. After hearing the sound several times, I started to wonder whether something supernatural was at work, but I hesitated to investigate.

In the morning, I heard the sound again while I was outside. When I turned to see what it was, I saw an apple rolling across the grass. Testing a theory, I picked up the apple and dropped it. The sound was identical to the one that had frightened me the night before.

The Death of Ghosts?Reports of ghosts bearing the news of deaths or disasters were frequent in the Victorian era, but they seem to be less common today. Researchers have offered a couple of possible explanations for this drop-off in supernatural messengers: People no longer report seeing the spirits of loved ones for fear of appearing crazy. Improvements in communication, like telephones and e-mail, have made it unnecessary for ghosts to intervene in human communication.

In daylight, looking at the fallen apples under a tree, the idea that the cabin could be haunted seemed silly. But dark nights and old buildings can cause even the most skeptical people to wonder about the existence of ghosts. According to a 2005 Gallup poll, more than a third of Americans believe that houses can be haunted, and about 32 percent believe specifically in ghosts [Source: The Gallup Poll News Service].

According to believers, a ghost is the spirit of a dead person that either has not moved on to the afterlife or has returned from it. The definition of "spirit" can vary. Some describe it as a person's soul, while others believe it is an energetic imprint that a person leaves on the world.

Humans have believed in -- or been skeptical about -- ghosts for thousands of years. They're even mentioned in the oldest known written work of literature, "The Epic of Gilgamesh." Ghost stories are part of most cultures' folklore, although the details vary considerably from region to region.

People describe ghostly encounters in lots of different ways. People see apparitions or strange lights, sense a presence in a room, hear noises or feel a sudden drop in temperature. They smell a deceased relative's favorite breakfast cooking in the kitchen or hear a favorite song playing while the stereo is off. Objects fall from shelves and doors open and close on their own. The electricity goes haywire, causing lights to flicker or televisions to turn on and off by themselves. Sometimes, people don't experience anything unusual at all, but they notice strange apparitions or shapes when they look at pictures they've taken.

Some ghost stories involve visible apparitions that are bound to specific locations or families. These ghosts often appear as a warning that someone is going to die. They aren't always human -- some take the form of animals. Similarly, some reports of ghosts involve apparitions that inform friends or family members of recent deaths or impending crises. Some paranormal researchers classify this as a form of telepathy rather than an actual ghost.

Other ghosts are reported to be the spirits of people who died violently or suddenly; they may re-enact their deaths or try to seek vengeance. For example, some people believe that North Carolina's Brown Mountain Lights -- flickering lights that appear on the slope of the mountain -- are the spirits of Native Americans who died in battle. Sometimes, ghostly reproductions of inanimate objects, like sunken ships or crashed cars, reappear after accidents or tragedies.

Image courtesy Stock.xchng Is it a ghost?

Then, there are the ghosts who are simply sticking around, either unwilling or unable to leave the Earth. Paranormal researchers often refer to these ghosts as earthbound spirits. An earthbound ghost may haunt a specific location, like its home, its favorite place to visit or the place that it died. It may be trying to pass a message to friends or loved ones, to complete a task that it started while alive or to hold on to its home or possessions. Some researchers and mediums claim to be able to encourage these sprits to let go of their ties to the Earth and move on to a spiritual realm.

For a lot of people, seeing, hearing or sensing a ghost is enough to prove their existence. But researchers have found several possible explanations for the phenomena most often attributed to ghosts. We'll look at them in the next section.

Orb PhotographySome paranormal researchers believe that photographs containing orbs, or unexplained spots of light, are signs of ghostly activity. Some describe orbs as a specific step in a ghost's manifestation [Source: Utah Ghost Research & Investigation]. Orbs are visible in pictures but invisible to the naked eye because the spirits react to infrared light from the automatic focus. Skeptics, however, think orbs have a physical cause, such as: The camera's flash reflecting off of dust particles or moisture in the air Water spots on the camera's lens Defects in digital cameras' sensors Developing or printing errors