Any geode might make us wonder: What geologic forces form these hollows lined with crystals? But the Pulpí Geode, discovered in an abandoned Spanish mine, takes wonder to a different scale. One of the world’s largest geodes, it’s an approximately 390-cubic-foot cavity whose walls bristle with imposing gypsum crystals, some nearly seven feet long. Now scientists are hoping to uncover how these colossal crystals developed.
They seem to have been made by a very specific recipe: a 250-million-year-old supply of the mineral anhydrite, a climate hospitable to crystal formation, and lots of water and time. In the resulting chemical soup, larger crystals may have cannibalized smaller ones to boost their own size, while swings in the local temperature could have accelerated the crystal growth even further.
Though key chapters remain incomplete, this otherworldly site now has a possible origin story. —Robin George Andrews