From the outside, a geode often looks like an ordinary rock. Crack it open, however, and you may discover a glittering crystal-lined chamber that has been forming for millions of years. But why are geodes hollow in the first place? Learn how these fascinating formations develop, why crystals grow inside them, and what determines whether they remain hollow or become completely mineral-filled.

In the quiet earth of southern Spain, hidden inside an abandoned silver mine, lies the Pulpí Geode — a room made not by human hands, but by the patient breath of the earth. Discovered in 1999 beneath the Sierra del Aguilón, this crystalline chamber is one of the largest known crystal geodes in the world, especially among selenite-lined types.