They are formed from groundwater saturated in calcium sulphate which, warmed by an intrusion of magma about a mile below, began filtering through the cave system millions of years ago. When, about 600,000 years ago, the magma began to cool, the minerals started to precipitate out of the water, and over the centuries the tiny crystals they formed grew and grew until 1985, when miners unwittingly drained the cave as they lowered the water table with mine pumps.
The crystals resemble giant icicles, the picture suggests it must be very cold inside the Cave of Crystals - but appearances can be deceptive. In fact, the temperature is a sweltering 112F, with a humidity of 90-100 per cent.This is why cavers wear protective suits and carry backpacks of ice-cooled air. Such conditions, and the fact that it takes 20 minutes to drive to its entrance through a twisting mine-shaft, haven't deterred would-be looters - one of the crystals bears a deep scar where someone has tried, and failed, to cut through it. But the cave has now been fitted with a heavy steel door, the better to preserve this beautiful wonder for generations to come.