Thríhnúkagígur is a dormant volcano near Reykjavík whose last eruption 4,500 years ago left a hollow magma chamber with bronze-and-indigo-colored walls. It's the only place in the world where people can climb into a volcano's plumbing system and explore the cavern that held sizzling molten rock before an eruption.

Since 2012, visitors have been allowed to enter Thríhnúkagígur's magma chamber via an open cable elevator that was built in 2010 for scientists. The chamber is about 700 feet (210 meters) deep, or more than double the height of the Statue of Liberty with the pedestal. Visitors are lowered to the bottom of the cave in a metal cage, and then they are free to roam the 33,600-square-foot (3,120 square meters) cave floor.

Volcanoes don't usually have empty magma chambers beneath them. Typically, during an eruption, there comes a point when the pressure in the chamber isn't high enough to eject any more liquid. The leftover magma slowly solidifies inside the chamber, filling the ground beneath the volcano.

Thríhnúkagígur is an exception, but researchers don't really understand why. The magma that fed the most recent eruption seems to have disappeared, and scientists think it may have been sucked back down into Earth's crust.

"It's like somebody came and pulled the plug and all the magma ran down out of it," Haraldur Sigurdsson, a volcanologist and professor emeritus of marine geology and geophysics at the University of Rhode Island, told Inside the Volcano, a company that organizes tours of Thríhnúkagígur.

The colors inside the chamber are attributed to microbes, but not much is known about the organisms that live there. Sulfur-rich gases likely painted some parts of the cave yellow and orange, while rocks falling off the walls formed the indigo and blue patches. Visitors occasionally see puffs of steam rising from corners of the chamber, but these are caused by water dripping onto lamps that illuminate the cave and are not signs of an impending eruption.

Evidence suggests Thríhnúkagígur has erupted three times over the past 50,000 years. The volcano presents three peaks at the surface — hence its name, which translates to "Three Peaks Crater." They are similar in height ‪—‬ about 115 feet (35 m) tall ‪—‬ and they form a line, with about 660 feet (200 m) between each peak.

The youngest peak dates to the eruption 4,500 years ago. At the top sits the opening where scientists and tourists start their journey into the heart of the volcano. The next peak in the line formed 5,000 years ago, and it is partly encircled with solidified lava from the most recent eruption, according to Inside the Volcano. The oldest peak formed 50,000 years ago, when Iceland was covered with a thick ice sheet. It is composed of hyaloclastite, a volcanic rock characterized by sharp, glassy fragments that appear when lava comes into contact with water and ice.

It's unlikely that Thríhnúkagígur will erupt anytime soon, even though the volcano sits within the Mid-Atlantic Ridge, a tear in Earth's crust where the Eurasian and North American tectonic plates are separating, according to Inside the Volcano.

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Sascha is a U.K.-based staff writer at Live Science. She holds a bachelor’s degree in biology from the University of Southampton in England and a master’s degree in science communication from Imperial College London. Her work has appeared in The Guardian and the health website Zoe. Besides writing, she enjoys playing tennis, bread-making and browsing second-hand shops for hidden gems.