LOS ANGELES, CA (Catholic Online) - In 1910, the Penoles mining company discovered what came to be known as "Cueva de los Espadas," or Cave of Swords. The cave was more than 250 foot hallway filled with gigantic gypsum crystals up to six feet in length. The cave, almost 400 feet underground, was opened for exploration and tourism, which led to the destruction of many of the crystals.

The Cave of Crystals contains giant selenite crystals, some of the largest natural crystals ever found. The selenite crystals were formed by hydrothermal fluids emanating from the magma chambers below. The cavern was discovered while the miners were drilling through the Naica fault, which they were worried would flood the mine.

Ninety years later, two miners working on an excavation tunnel 1,000 feet below the Earth's surface discovered another, much larger, cave. Here, the crystals measured over 36 feet with a single crystal weighing approximately 55 tons. These were the largest crystals that anyone had ever seen in the cave. The cave came to be known as the "Cueva de los Cristales" or Cave of Crystals.

Millions of years ago, volcanic activity filled the mountain with anhydrite, a waterless form of gypsum. As magma eventually cooled, the anhydrite began dissolving, enriching cave waters and allowing gigantic crystals to form. The size of the crystals has no limit and given enough time will grow to even larger proportions.

There are very few other places in the world where such crystals have been so well preserved in their purest of forms.