I was amazed to come across an article and video documentary from National Geographic about the so-called 'Cave of the Crystals' at Naica in Mexico.
The article describes them as follows:
The limestone cavern and its glittering beams were discovered in 2000 by a pair of brothers drilling nearly a thousand feet below ground in the Naica mine, one of Mexico's most productive, yielding tons of lead and silver each year. The brothers were astonished by their find, but it was not without precedent. The geologic processes that create lead and silver also provide raw materials for crystals, and at Naica, miners had hammered into chambers of impressive, though much smaller, crystals before. But as news spread of the massive crystals' discovery, the question confronting scientists became: How did they grow so big?
. . .
In the presence of such beauty and strangeness, people cast around for familiar metaphors. Staring at the crystals, García decided the cavern reminded him of a cathedral; he called it the Sistine Chapel of crystals. In both cathedrals and crystals there's a sense of permanence and tranquillity that transcends the buzz of surface life. In both there is the suggestion of worlds beyond us.
There's more at the link.
Here's the video documentary about the caves. It's 1½ hours long, but if you have the time, it's worth watching.
I wonder how many more hidden wonders like this await discovery?