Cave of the Crystals in Mexico is extremely hot with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors. Without proper protection people can only endure approximately ten minutes of exposure at a time. A group of scientists known as the Naica Project have been heavily involved in researching these caverns. Continue reading for more mind-blowing natural caves you can actually visit.