Wieliczka Salt Mine

The Wieliczka Salt Mine, near the Polish city of Krakow, is a salt mine that has been in continuous operation since the 13th century, and still is producing table salt today. The mine stretches to a depth of 327 meters and is more than 300 km long.  In addition to its purpose as a mine, Wieliczka features a oturist route lined with statues of historical and mythical figures (all of them sculptured out of salt by miners), chambers and chapels lined with salt crystal chandeliers, underground lakes, and exhibits showing the history of salt-mining.

Cueva de los Cristales de Naica

The Crystal Cave was accidentally discovered in 2000 by miners working in the silver and lead mine at Naica, Mexico. It lies almost 300 meters (900 feet) below the surface of the Earth and it contains the largest crystals known in the world, by far. The largest crystals are over 11 meters long (36 feet) and weigh 55 tons.  The crystals themselves are made of selenite which is crystallized gypsum, the same material used in drywall construction. Except these crystals formed over a span of about half a million years in a hot water solution, saturated with minerals. The temperature inside the cave remained very consistently hot for the entire time the crystals were growing.