Nothing in the world can compare to Cueva de los Cristales, or Cave of Crystals, found in Mexico. The world's largest, broadest, and thickest crystals measure over tens of meters in length, and they are so clear, so luminous, and so huge that they just seem out of this world.
The cave is, of course, not open for visiting, and even for those who get inside it to study or take professional pictures, there is a 20 minute limit, for their own protection. The humidity there gets to 100%. For hundreds of thousands of years, groundwater saturated with calcium sulfate filtered through the many caves at Naica, warmed by heat from the magma below. As the magma cooled, water temperature inside the cave eventually stabilized at about 136°F. At this temperature minerals in the water began converting to selenite, molecules of which were laid down like tiny bricks to form crystals.