It's a question at par with "What happens when we die?" and "Why are we all here?"
One of those eternal human mysteries that science has been unable to find a definitive answer to: What is a near death experience and what causes it?
Near death experiences or NDEs are defined as as feelings that sometimes occur when a person is nearly dead or has been clinically declared dead. They're typically accompanied by out-of-body experiences, journeys through dark tunnels, flashes of light, the awareness of being dead, feelings of peace and calm, meetings with deceased relatives, life reviews and the disappearance of the fear of death.
Often, people report hearing or seeing things while clinically dead that they are able to confirm upon resuscitation.
Despite their relatively frequent rate of occurrence -- in one study, 1 in 10 cardiac arrest survivors reported having NDEs -- little research has been done on them. In fact, advances in medical science mean that more and more people are being resuscitated after being declared dead, which in turn has led to an increase in the number of reported NDEs.
In 2008, Dr. Sam Parnia, one of the foremost experts on consciousness and death, launched the world's largest study on near death experiences by deciding to interview nearly 1,000 survivors of cardiac arrest. The results will be released next year.
Interpretations of NDEs vary wildly. The more religious view, of course, is that NDEs are evidence of the existence of life after death. There are several well-known proponents of this theory -- from Colton Burpo, the 11-year-old who recently wrote the bestselling "Heaven Is For Real" about his NDE at age 4, to Don Piper, the minister who wrote about his 1989 NDE in "90 Minutes In Heaven." Proponents of this theory point to the universality of NDEs -- their features remain the same, regardless of where in the world they occur -- as proof of their spiritual, non-scientific nature.
But the scientific community, too, differs on its interpretation of NDEs. The most common explanation is the "dying brain hypothesis," which is the idea that when the brain is under stress, it releases a flood of neuro-chemicals that create flashes of light, peace and calm.
In April 2010, researchers in Slovenia analyzed NDEs in cardiac arrest patients. They found patients who reported an NDE also had much higher carbon dioxide levels than those patients who did not. The study suggests this increase in carbon dioxide could be what triggers NDE symptoms.
Yet another study, conducted in 2009 by University of Kentucky researchers, asserted that NDEs are caused by a "blurring of sleeping and waking states." Others, like Dr. Parnia point to the fact that the brain stays alive and consciousness continues several hours into the process of dying.
Still, several questions remain unanswered. Why do some people report seeing things -- verified later -- when their eyes are closed? If NDEs are caused by simple chemical excesses, why do people report being so fundamentally changed by them? Perhaps Dr. Parnia's study will give us a definitive answer. Perhaps we'll never really know. And maybe it's best that way.
The following lists some of the more well-known stories of people who say they've had a near-death experience, along with researchers who've studied them.