Cave of the Crystals

Pictures inside of the cave:
https://imgur.com/gallery/EwG0X

BBC video inside of the cave:
https://www.youtube.com/watch?v=HeiMfLmJtzk

BBC article accompanying the video:
http://news.bbc.co.uk/2/hi/8466493.stm

How Stuff Works:
https://science.howstuffworks.com/environmental/earth/geology/mexico-giant-crystal-cave.htm

Book of Knowledge:
https://en.wikipedia.org/wiki/Cave_of_the_Crystals