I have heard that a fire in a cave or under an overhang can cause a rock collapse, potentially injuring the occupant. Is this a real risk?
Is this a real risk?
However, I would have thought it's a much higher risk with soft rock, such as sandstone, which could easily crack and break apart due to heat. I'd imagine the risk would be lower with something like granite, but I'm still not sure I'd risk it unless the cave was very large and well ventilated.
There is another problem with making a fire in a natural cave: the smoke propagating through the cave can severely affect various cave-dwelling species, particularly bats. That is especially bad during winter, when smoke and warm air from the fire can wake up the bats and drive them out of the cave, causing them to freeze or starve to death.
The general rule is that the rock can't get hot. When you heat the rock, the water inside evaporates and can crack the rock. If you're just making fire to warm yourself or cook something (a steak, not the whole beef) it will be OK. When the fire is moderate, the rock shouldn't even get warm.
You shouldn't also throw bigger stones into fire. They can explode.
The example cited by berry120 was something like the-biggest-fire-in-the-world-challenge... When you want to fire something big in the cave, I would be afraid of CO intoxication in the first line, however.
The only time I lit a fire in a cave we had to evacuate due to smoke and a billion spiders dropping from the ceiling.
This isn't a 'danger' for me in the UK - we do not have deadly spiders. Elsewhere this might be a significant danger.
Much better to light it just outside :-)
Use a reflector fire to bounce the heat inside the cave and funnel the smoke across the cave mouth - much the same as you would setting up a lean-to type shelter.
At first, I was going to down-vote the question, because "I have heard..." struck me as relying on urban myth. But I realized I didn't have the Rep yet on this site to do so...
Then coincidentally, I came across this sad story from 2009:
A 16-year-old boy was crushed to death by falling stones in a cave which had probably been unsettled by the heat from his camp fire, an inquest heard.
Geology expert Andrew Bowden told the inquest that a pre-exisiting weakness in part of the 1,000-year-old cave would have combined with the heat of the fire to cause about 200kg of rock to fall away from an overhang.
[Edit: This refers to the same story linked by Berry above]
But under normal circumstances, I suspect Carbon Monoxide poisoning and/or smoke inhalation is the bigger risk...