Mexico's Cave of Crystals is one of these places. Discovered by two miners in 2000, and drained of it's heavy-mineral-content water in 2007, the cave is home to enormous gypsum crystals, some as long as 11 meters (36 feet).
The Cave of Crystals is a horseshoe-shaped cavity in limestone rock about 30 feet (10 meters) wide and 90 feet (30 meters) long.
Volcanic activity that began about 26 million years ago created Naica mountain and filled it with high-temperature anhydrite gypsum (giant shards of which are pictured above).
When magma underneath the mountain cooled and the temperature dropped, the anhydrite began to dissolve. The anhydrite slowly enriched the waters with sulfate and calcium molecules, which for millions of years have been deposited in the caves in the form of huge selenite gypsum crystals.
"There is no limit to the size a crystal can reach," geologist Juan Manuel García-Ruiz said.
Here is another webpage detailing further this amazing cave.
