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Half of the world’s 700 mud volcanoes—oozing, gurgling mounds of once-subterranean sludge and methane—are located in Azerbaijan, concentrated around Gobustan National Park along the coast of the Caspian Sea.

Unlike lava-spewing igneous volcanoes or the whiffy bubbling mud of Rotorua, the contents of Gobustan’s mud domes are cold. Their main danger is in their unpredictability—a buildup of pressurized gas in the cone can be released without warning, causing possible asphyxiation, triggering a jet of fire, and drawing a torrent of fast-flowing mud from the volcano.

In 2001 gas seepage caused a mud volcano near Azerbaijan’s capital of Baku to explode. The mound shot flames hundreds of feet into the air, burned for three days, and filled the sky with mud and black smoke.

If you’re pondering a visit to the gurgly hills of Gobustan, make sure to also see the UNESCO World Heritage-listed rock art in the area, which dates back tens of thousands of years.

Visit Atlas Obscura for more on the mud volcanoes of Azerbaijan.

More mud in which to wallow:

• The central mosque of this Mali town is built entirely from mud
• Myanmar’s muddy eruptions are less deadly and more holy than actual volcanoes
• This city, built from mud, is known as the Manhattan of the desert