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NGSS: Core Idea: ESS2.A

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TEKS: Science: 3.3A, 4.7B, 5.5C, 6.10B; ELA: 3.7E, 4.7E, 5.7E, 6.6E

Crystal Cavern

Peek inside a giant rock in Spain filled with crystals 

JORGE GUERRERO/AFP via Getty Images

A geologist inside the Pulpí Geode

As you read, think about how you might feel if you were inside the Pulpí Geode.

Jim McMahon

From the outside, geodes 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 geologist at the National Museum of Natural Sciences in Madrid, Spain, who has studied the geode. 

They may look like regular rocks on the outside. But crack a geode open. Inside, its walls are lined with sparkling crystals. Many geodes could fit in your hand. But there’s a geode in southern Spain near the city of Pulpí that is 8 meters (26 feet) long! 

The Pulpí Geode is one of the largest ever found. And it recently opened to the public! Visitors must trek 50 meters (164 feet) underground to see it. They walk through the tunnels of an old mine. Then they squeeze through the geode’s 1 meter (3 foot)-wide opening.

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

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 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.

The giant geode is old. Scientists think it began forming millions of years ago. Mineral-rich rainwater seeped below ground. It ate away at rock. That formed hollow spaces. The water slowly evaporated. It changed from a liquid to a gas. It left behind a mineral called gypsum. 

Gypsum particles stuck together as the water evaporated. They arranged themselves in orderly patterns, forming crystals. These crystals grew larger and larger over time. Today, some crystals are 2 meters (7 feet) long! Gypsum crystals that large are rare, says García Guinea. The only others that big are found in Mexico and Chile. 

Efrén Cuesta found the Pulpí Geode in 1999. He’s a mineral collector. He was exploring the mine. Other collectors made the journey to see the geode over the years. That eroded the geode’s entrance. It wore down, making it easier to pass through.

A Stunning Sight

Tomekbudujedomek/Getty Images

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.” 

The Pulpí Geode’s gypsum crystals are fragile. The air inside the geode has to stay cold and humid. That keeps the crystals from falling apart. García Guinea and other scientists are working to protect the geode. One way is to limit how many people can view it at a time.

Visitors to the mine are led down a spiral staircase. The rock around it is full of holes and 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 stick their bodies halfway through the opening to peek inside. García Guinea was nervous when he first entered. But he was also excited. “My heart raced because I was seeing something incredible,” he says.

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