There is a scene, in World's Awesomest Movie™ The Core, where our heroes are in a spaceship drilling through the Earth's crust (don't ask) and they burst into the center of a gigantic geode (really, don't ask). There are huge purple amethyst spears everywhere, and when I saw this in theaters, I laughed so hard I nearly choked to death on the grapefuit-juice-and-vodka I was sipping from my smuggled-in flask. Turns out this scene was not as implausible as I thought. Science is amazing!
Ladies and gentlemen, I present to you: The Crystal Cave of Giants from Naica, Mexico:
These are gypsum crystals in a silver mine. For hundreds of thousands of years, conditions in this hidden chamber were so protected and stable that normal crystallization processes resulted in enormity. Mining processes removed the fluid, though the chamber remains superheated by surrounding magma (hence the suits, which are filled with ice).
The explorer's own description: "Actually going inside, wearing the suits and exploring the cave was a dream come true. I've never seen such a spectacular place. It was like setting foot on a new planet. Many of the crystals were so large that I couldn't even wrap my arms around them and the terrain was so difficult to walk on that we had to be extremely cautious not to slip and fall. Doing so would could get you impaled on a sharp crystal and would require a dangerous and difficult rescue."
Cue the action scene!