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JORGE GUERRERO/AFP via Getty Images

Crystal Cavern

Peek inside a giant rock in Spain filled with crystals 

By Jeanette Ferrara
From the March 2021 Issue
Other Focus Areas: Plate Tectonics and Landforms
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As you read, think about how you might feel if you were inside the Pulpí Geode.

Woman sitting inside a cavern, surrounded by giant crystals

JORGE GUERRERO/AFP via Getty Images

A geologist inside the Pulpí Geode

Map of the world showing the location of Pupí, Spain

Jim McMahon

From the outside,  look like regular round rocks. If you crack one open though, you’ll find its walls are lined with sparkling crystals. Many geodes are small enough to fit in your hand. But in southern Spain near the city of Pulpí, you can find a geode that’s 8 meters (26 feet) long! 

Known as the Pulpí Geode, it’s one of the largest crystal-filled rocks ever discovered. And it recently opened to the public! To see it, visitors must trek 50 meters (164 feet) underground through the tunnels of an abandoned mine and squeeze through the geode’s 1 meter (3 foot) wide opening.

The experience is otherworldly, says Javier García Guinea. He’s a at the National Museum of Natural Sciences in Madrid, Spain, who has studied the geode. 

A red rock entrance carved into a a green, scrub-covered hillside

CARLOS BARBA/EPA-EFE/Shutterstock (Mine)

To reach the geode, visitors must enter an abandoned mine in southern Spain.

Rare Rock

Scientists think the giant geode began forming millions of years ago. As -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 , or wore down, the geode’s entrance, making it easier to pass through.

A Stunning Sight

Geode filled with purple crystals

Tomekbudujedomek/Getty Images

Small Geode

The Pulpí Geode’s gypsum crystals are fragile. To keep them from falling apart, the air inside the geode has to stay cold and humid. García Guinea is part of a group of scientists who are working to protect the crystals. This means limiting how many people can view the geode at a time.

Visitors are led down a spiral staircase through the mine, which is full of tunnels. “The rock is like Swiss cheese,” says García Guinea. A pillar at the center of the stairs supports the rock. That’s important because the area is prone to earthquakes. 

Unlike scientists, tourists aren’t allowed to fully enter the geode. But they can squeeze half their bodies through the opening to peek at the crystals. When García Guinea first entered, he was anxious—but also exhilarated. “My heart raced,” he says, “because I was seeing something incredible.” 

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