Gypsum megacrystals...

beyond belief are shown in this slide show about the Naica Crystal Cave in Mexico. Silver miners broke into this deep fluid-filled chamber more than eight years ago, and once the grotto was drained, found this astonishing mineral deposit. This video includes an excellent interview with a NASA scientist that is studying the unique occurrence.

Link to technical abstract describing the geochemistry of these enormous gypsum/selenite crystals.