Giant Crystal Cave, Mexico

Giant Crystal Cave was discovered nearly 1,000ft below the Chihuahua Desert in Mexico. The main chamber contains giant selenite crystals, some of the largest natural crystals ever found. The cave's largest crystal found to date is 12 m (39 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight. The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity.

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.