Cave of Crystals, Mexico
The Cave of Crystals in Mexico was first discovered in 1910 by Spanish miners. Its main chambers contain some of the largest and most beautiful natural crystals in the world. Despite this, without proper protection people can only withstand the heat of its extreme temperatures for ten minutes, causing limited exploration.
Spelunkers had to wear refrigerated suits and heavy protective gear to venture into the site when studies were taking place, but even then, high water levels and the intense heat deterred them from delving too deep. Now, all exploration and research has been stopped to allow the water levels to rise and the crystals to blossom once more.