Spontaneous human combustion or SHC is a phenomenon where a living human body without an apparent external source of ignition starts to ignite from within spontaneously. SHC is not very well studied or documented by modern science yet as there are not many reported cases to investigate. However, it is a valid phenomenon that is continuing to baffle medical experts and scientists. There are experts on both sides of the fence supporting or rejecting SHC. According to Wikipedia, there have been more than 200 cases documented over a period of 300 years.
The thing that is astounding apart from the spontaneous ignition is that the furniture and other objects proximal to the victim is not damaged or burnt by the high temperatures. Damage is limited only to the victim’s clothing, floor area and furniture. Anyways, SHC is a modern day mystery that is yet to be fully studied.
Although it seems that SHC is a modern day mystery, actually, there is a reference to it in the Bhagavatam. Srimad Bhagavatam is an ancient book spoken 5000 years ago and documented as such. In it, there is a reference to SHC and the nature of it. Please see below.
The material body is a combination of earth, water, fire, air and ether. There is already fire within the body, and our practical experience is that the heat of this fire sometimes increases and sometimes decreases. The fire within the bodies of the sons of Sagara Mahārāja became so much hotter that all of them burned to ashes. The fire's increased heat was due to their misbehavior toward a great personality. Such misbehavior is called mahad-vyatikrama. They were killed by the fire of their own bodies because of insulting a great personality.