Stretching to the horizon, the gray-white earth rises in peaks like a well-whipped meringue petrified into mud and rock.

A keen wind whips these otherworldly hills, while the land underfoot is cracked and scaly.

This could be Mars. But it's Azerbaijan, the mud volcano capital of the world.

Oil-rich region

There are more than 400 volcanoes in this oil-rich country balanced between Europe and Asia, with more being discovered all the time.

Ten years ago, a 12,000-hectare State Nature Reserve was founded on the Absheron Peninsula near the capital Baku.

In July 2018, scientists from Azerbaijan's Ministry of Ecology and Natural Resources took CNN Travel on an off-road tour.

Clattering up the precipitous slopes in an off-road vehicle is like a firmer version of the dune-bashing safaris popular next door in the Middle East. But the most thrilling sights are still to come.

We start our tour beside the mouth of Deveboynu volcano, or "Neck of the Camel." The land around is parched, but inside the crater, a muddy slurry -- called mud breccia -- is bubbling gently.

The hills may look peaceful, but below this humped, reptilian landscape, the Earth is breathing and turning like a slumbering beast.

Maureen O'Hare/CNN

Where tectonic plates collide

Mud volcanoes are usually found in subduction zones -- the parts of the globe where tectonic plates collide, sometimes causing earthquakes, tsunamis and violent eruptions.

Pockets of subterranean gas force their way to the surface, belching out methane and a muddy mix of water and oil.

Mud volcanoes never grow to the size of regular volcanoes. Most are small, ever-changing landforms, often accompanied by peculiar sculptures formed by the spewed out clumps of clay.

These whimsical shapes, says Jeyhun Pashayev, director of the reserve, are "like a picture made by a child."

Strange beauty

"When there are mud volcanoes, it means there are carbohydrogen resources in the deep," explains Pashayev. "As you see, there are lot of mud volcanoes in Azerbaijan which means there are a lot of oil resources."

The arid lands of the Absheron Peninsula might not be as fresh and lush as the mountainous north, but they have their own strange beauty.

This is the region richest in natural resources: both for oil and for the mud breccia, which is used in the chemical and construction industries.

"About 20 of the 50 minerals used in cement" are to be found in the volcanic mud, says Pashayev -- which meant that before the reserve was signed into existence, people regularly stole mud from the site for home building projects.

As we set out on the road again to our next volcanic field, Pashayev gestures towards a huge factory on the horizon, with a large hill of mud to its side. "This, Garadagh, is the richest part of Baku. Around 90% of the stones are being produced here for building the houses. Depending on the quality, it costs from 50 cents to 3 manats ($1.77) per stone."

Maureen O'Hare/CNN

Mud-bathing

Unlike regular volcanoes, there's no magma or lava, so the liquid mud isn't hot -- instead it usually keeps to a temperate 22/23 C (72 F).

That balminess means that some mud volcanoes are suitable for bathing.

Volcanic clay is sometimes used in the treatment of skin conditions, as well as rheumatism and diseases related to the nervous system

Pashayev explains that during the Soviet era, the volcanic mud was sometimes used to create casts to treat broken bones.

"Also, you can get super results using the mud volcanoes for the skin," he claims. "You can use [the mud] and get good results, even if you have scars. If the cosmetology doctor can't disappear the scars, mud volcanoes can do this!"

Radioactivity

However, not every gray puddle is skin-friendly.

"Because it comes from deep," says Pashayev, some volcanoes "come with radioactive material. It's not a joke."

Pashayev's team monitors the levels of radioactivity regularly, with safety certification being awarded on an annual basis.

The subterranean world has fascinated people around the globe for millennia, and the strange, violent and seemingly inexplicable behavior of mud volcanoes has long captivated the Azerbaijani people.

The country is known as the Land of Fire -- while Baku is the City of Winds -- and some of the gas leaks in this oil-rich region are constantly on fire, such as at the perpetually burning mountainside of Yanar Dag.

It's believed that the ancient Zoroastrian religion, which centers on fire worship, was closely connected to this phenomena.

In 2001, Lokbutan volcano some 15 kilometers from Baku erupted, sending flames close to 50 feet in the air.

'A mystic thing'

Maureen O'Hare/CNN

"Mud volcanoes are a mystic thing," says Pashayev. Some years ago, a Danish sound archivist came to the reserve "with a huge device like a piano" to record the sounds from within the volcano.

What Pashayev heard shocked him. "I swear, I thought that this is Hell. I heard human noises -- really. Imagine thousands of people, crying 'help us!' from the deep."

With a similarly evocative turn of phrase, Pashayev also compares the wind sighing through a mud sculpture to the sound made "when you cut the head off a cow" -- a common sight on the backstreets of Baku until butchery regulations were changed earlier this year.

And if the Azerbaijani landscape seems otherworldly, that could be because conditions here aren't dissimilar to Mars.

A number of possible mud volcanoes have been found on Mars, suggesting the presence -- at one time -- of water, and therefore the possibility of life.

'Millions of snakes'

"There are six types of mud volcano," explains Pashayev. "Active. Not active. Oil-producing. Underwater. Island mud volcano. And destroyed mud volcano."

There are around 140 underwater mud volcanoes in Azerbaijan and there are six island volcanoes under the reserve's jurisdiction -- although explains scientist Anar Baghiriv, the team lacks the resources to visit them.

But on one of the island volcanoes, Pashayev explains, the strangest sight isn't the bubbling mud. "From 100 meters, you see that the island is shaking. But when you go close, you see millions of snakes, can you imagine? Millions."

So covered is the island with sea snakes, adds Baghiriv, "You cannot find a place to put your foot."
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What lies beneath

Pilpili Gardag is the last stop on our tour. Once we're ushered past the security guard, it's a steep walk uphill to the most dramatic volcano yet.

The slick surface of the mud glistens in the searing July sunshine, with huge gray bubbles forming and then bursting with a satisfying plup.

Mud volcano eruptions vary enormously, from those that quietly, continuously pour out slow streams of slurry, to explosive eruptions that expel thousands of cubic meters at once.

In Azerbaijan, says Pashayev, we have "different sizes, different views, different sculptures. We have all the types, all the forms."

The Guinness Book of Records lists the world's largest mud volcano as being an Azerbaijani one, measuring one kilometer across the base and several hundred kilometers in height.

At Pilipili Gardag, the volatile lunar landscape, with the sun bouncing white off the bleached-out clay and the summer heat sliding towards 40 C (104 F), has an uncanny majesty.

But the real mysteries, the churning, living wonders, are in the many shifting layers beneath our feet.

CNN

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