Cave of Crystals (Mexico)
Credit: Alexander Van Driessche / Wikipedia

Cuevas de las Cristales looks like something out of either a fairy – or cautionary – tale. Massive, perfectly symmetrical, crystalline formations cluster like iron filings, pouring out of the ground and walls in every direction. The crystals formed due to the cave's position atop a magma chamber, which heated groundwater to create perfect conditions for suspended minerals to snap into place. The cave's proximity to volcanic activity also makes it oppressively hot, warmer still at its lower depths. Temperatures exceed 110 degrees and humidity approaches 100 percent, so spelunkers have to wear specialized cooling suits.