A geode is a rock with a hollow space inside that is lined with crystals. Many geodes are small enough to hold in your hand. But this geode in Pulpí, Spain, is big enough for people to stand inside. Discovered in 1999, it was opened to the public last year.
The giant geode began forming millions of years ago as hot water seeped underground. The water slowly evaporated, leaving behind a clear mineral called gypsum. The gypsum gradually formed crystals that resemble huge jagged pieces of ice.