Spontaneous combustion occurs when an object — in the case of spontaneous human combustion, a person — bursts into flames from a chemical reaction within, apparently without being ignited by an external heat source [source: National Parks Service].
The Danish anatomist Thomas Bartholin has been credited with penning the first written account of spontaneous human combustion. In 1663, he described how a woman in Paris "went up in ashes and smoke" while she was sleeping. The straw mattress on which she slept was unmarred by the fire. In 1673, a Frenchman named Jonas Dupont published a collection of spontaneous combustion cases in his work "De Incendiis Corporis Humani Spontaneis" [source: Reville].
The hundreds of alleged SHC accounts since that time have followed a similar pattern: The victim is almost completely consumed, usually inside his or her home. Coroners at the scene have sometimes noted a sweet, smoky smell in the room where the incident occurred [source: Endeavour].
There's something peculiar about many of the photos that document suspected SHC victims. In a number of these shots, the corpse's torso and head are charred beyond recognition, but some extremities remain intact, with the hands, feet, and/or part of the legs being apparently unburned. Also, the room around the person may show minimal fire damage — though a greasy residue is sometimes left behind on furniture and walls [source: Nickell].
Reports have also been made about spontaneous human combustion victims who didn't simply burst into flames. These individuals are said to have developed strange burns on their bodies with no obvious source. And not every person who supposedly caught fire has died — some people say they've experienced SHC and then lived to tell the tale [source: Lewis].