Deep inside Naica mountain, 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.

• More Photos in the News
• Today's 15 Most Read Stories
• Free Email Newsletter: Focus on Photography