Scientists think the giant geode began forming millions of years ago. As mineral-rich rainwater seeped underground, it dissolved the rock, forming hollow spaces. The water then evaporated, or changed from a liquid to a gas. It left behind a mineral called gypsum.
Over time, as more water evaporated, gypsum particles attached to each other in orderly patterns, forming larger and larger crystals. Today, some crystals are 2 meters (7 feet) long! Gypsum crystals that large are extremely rare, explains García Guinea. The only others are found in Mexico and Chile.
The Pulpí Geode was discovered in 1999 by mineral collector Efrén Cuesta as he explored the abandoned mine. Over the years, other collectors made the journey to the geode. That eroded, or wore down, the geode’s entrance, making it easier to pass through.