It was only luck that led to 26-year-old Matthew Head being rescued from this disused lead mine earlier this week. Head, who had been missing for 11 days, was heard by a group of schoolchildren out on an evening walk in Somerset's Mendip Hills.
He had fallen about 40ft and suffered lower limb injuries, hypothermia and dehydration.
But he had survived. The question is, how? And how much longer could he have survived? And what are the essentials for survival?
"The key is water," says physiologist Dr Peter Tatham. "It's absolutely crucial to drink. Otherwise you will be critically ill within three or four days."
The cave rescue team that freed Head estimated that he had been in the mine for around four days. He might have been able to help himself by lapping up any moisture he could find, and could have been assisted in this by any rainfall that may have penetrated into the shaft where he was lying. Still, he was clearly dehydrated.
"Humans lose water in different ways which we can't prevent," says Tatham. What we call insensible water loss occurs both through perspiring - even if we can't see the sweat - and also losing vapour through your breath. You've also got to allow for the fact that you can't stop the body from urinating - it needs to get rid of the salt in the body - and that will use up a minimum of 700ml. I think you need to drink at least 1.7 litres a day to keep you alive, although if you have access to food, you will be getting some fluid through that."
The weather can also come into play. "If it's hotter, you will get more insensible water loss, particularly through sweating," says Tatham. "But being cold isn't necessarily of benefit either. If it's cold, hypothermia gets you. You'll start shivering and lose energy, which you won't be able to replace."
First-aid courses traditionally offer an easy-to-remember mantra when it comes to survival. They suggest that humans can survive three minutes without oxygen, three days without water and three weeks without food. The real timings may vary, but these figures do give a realistic comparison of how important the fundamentals are.
"We once found a man on a mountain top who hadn't had anything to drink for five days," says Willie Marshall, chairman of the mountain rescue team of Scotland. "But I don't think he would survived for much longer than that. He would soon have needed water, even though he was probably OK without food. It's all down to the way the body operates. We carry food reserves in our body, and can convert fat to energy, which means we can survive for longer without actually eating anything."
But making the most of the little food or water you have is not the only issue. It may help you to survive physically, but surviving psychologically brings its own problems.
"People are remarkably resilient," says Dr James Thompson, senior lecturer in psychology at UCL. "We don't like to think how horrible it must be when these situations happen, but only a relatively small number of people give up. Generally, people in these circumstances do things which are rational and which will help them. They make a noise and try and attract other people's attention. They also have times of dreadful despair and inability to cope, but they usually keep on trying. They may break down and think they can go no further, but after a while, perhaps after a sleep, they have another go."
Having no food and especially no water may be your most immediate problems, but being trapped in a small space can be traumatic too. Psychologically, trying to keep yourself going in a place that may be pitch black, is similarly unpleasant.
"You don't need light to survive," says Thompson. "But that's in the sense of survival meaning getting you out with your heart still beating. If you're suffering from sensory deprivation and you're frightened, then that can be psychological torture."
Of course, the problems don't end when you're out of danger. Escaping from that mineshaft, mountain or pile of rubble, does not mean that you're back to normality.
"There's lots of evidence that, even if you have had an extraordinary escape, it does not give you immunity to the psychological after-effects," says Thompson. "You can be a hero, but still suffer from trauma afterwards. They are two completely different states of mind."