In a Mexican zinc mine, gypsum crystals over 30 feet long have been found. They need temperatures over 120F and submersion in water to grow, so it’s a rare find. One would deduce that such conditions existed for a long time in this particular cave.
The Giant Crystal Project Site here.
Brief note: It’s not the end of the world, National Geographic, so please just focus on the science part of your mission, and let me think for myself and maintain my cherished skepticism: The National Geographic-Marching To The Eco-Drumbeat