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From the mountains of China to the warm waters of the Caribbean, sinkholes pockmark the surface of our planet. These phenomena – which almost make the Earth look as if it’s being devoured from within – resemble vertical caves and are home to a plethora of exciting discoveries, including unique floating islands, strange stalactites, rare plants and endangered animals. Mysterious-seeming as well as awe-inspiring, these natural occurrences attract scientists, tourists, BASE jumpers and divers from all over the globe. Here’s a look at 10 of the most incredible sinkholes in the world.

10. Minyé Sinkhole – New Britain, Papua New Guinea

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Situated on New Britain island in Papua New Guinea, the Minyé sinkhole drops to an astonishing depth of 1,673 feet (510 meters). New Britain is characterized by volcanoes, limestone mountains, raging underground rivers and tropical rainforest. Yet the island’s vibrant terrain disappears abruptly into a massive depression, which at its deepest point could hold one and a half Eiffel Towers with room to spare.

The Minyé sinkhole is also 1,148 feet (350 meters) wide, meaning four Boeing 747 jumbo jets lined end to end would easily fit across it, with almost enough space for a fifth. A ferocious river system flows along the bottom through an extensive cave system containing one of the most gigantic caverns in the world. It is thought that the Minyé hole was formed due to the effects of underground rivers and heavy rainfall, which eventually wore away the limestone rock and led to the collapse of the surface layer.

9. Sima Humboldt – Bolivar, Venezuela

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The vertical walls of the gaping Sima Humboldt sinkhole break up the thick carpet of jungle covering Cerro Sarisariñama, a flat-topped mountain in Bolívar, Venezuela. The sinkhole is 1,030 feet (314 meters) deep, and its width varies from 1,155 feet (352 meters) wide at the rim to 1,647 feet (502 meters) wide at the base.

What makes Sima Humboldt so unusual is that it was formed in quartzite sandstone, which is naturally highly resistant to erosion. Oddly, there is no river at the base that could have contributed to the sinkhole’s formation, so it is likely that it was carved out of rock very slowly through a process of chemical erosion and collapse. Thanks to the sheer sides of Sima Humboldt, the forest and animals below have been isolated from the plateau peak for many, many years, creating a truly unique ecosystem.

8. Dean’s Blue Hole – Long Island, Bahamas

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Dean’s Blue Hole is located on Long Island in the Bahamas, in the gorgeous waters of Turtle Cove. Interestingly, the hole is the world’s deepest known saltwater sinkhole. Such phenomena often referred to as “blue holes,” since they’re filled with or under water, and this particular blue hole is 663 feet (202 meters) deep. At its entrance, it is up to 115 feet (35 meters) in diameter, but this width increases dramatically further down.

The sinkhole is a free diver’s paradise, with one visitor saying that the experience feels like “swimming and floating through outer space.” Blue holes are common in the Bahamas and are the subject of many local legends. In fact, according to CBS News, some islanders believe that Dean’s Blue Hole was created by the Devil himself. These folk fear that if you swim in or even float over it, you will be sucked downwards, never to be seen again.

7. Sima de las Cotorras – Chiapas, Mexico

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Located in Chiapas, Mexico, Sima de las Cotorras – meaning “Sinkhole of Parrots” – is an impressive 520-feet (160 meters) wide, 460-feet (140 meters) deep perforation in the landscape. It is well known for the myriad green parakeets that seek refuge in the trees at its base. Other birds and small animals – including hummingbirds, orioles, woodpeckers, coyotes, anteaters and iguanas – also live in the isolated forest at the bottom.

Like many other similar sinkholes in the area, Sima de las Cotorras was most likely formed out of the limestone rock by the gradual dissolving power of an underground river, such as the nearby Río La Venta. Another unique feature of this particular sinkhole is that 230 feet (70 meters) down from the rim there are 46 rock paintings, which experts believed were painted by ancestors of the native Zoque people as many as 10,000 years ago.

6. Bimmah Sinkhole – Muscat, Oman

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The Bimmah sinkhole is situated in Hawiyat Najm (“Meteor Fall”) Park in the region of Muscat in northern Oman. Gorgeous turquoise-colored water fills the 65-foot (20-meter) deep, 131-foot (40-meter) wide sinkhole, and an underwater tunnel links the hole to the sea – which may be the reason the water is such a mesmerizing shade of blue. The Bimmah sinkhole has become a popular tourist destination: some visitors enjoy swimming in the water watching hundreds of little fish dance about their toes, while others leap from the rim and land in the pool below with a splash. Although the sinkhole was formed by a natural process of limestone erosion and subsidence, local legends claim – more colorfully – that it was created by a falling meteor.

5. Xiaozhai Tiankeng – Chongqing, China

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Xiaozhai Tiankeng, also referred to as the “Heavenly Pit,” is a double-nested sinkhole in the Chinese district of Chongqing. This incredible cavity in the Earth’s crust is the largest sinkhole in the world. The combined depth of Xiaozhai Tiankeng’s nested holes is a whopping 2,172 feet (662 meters), and the yawning depression is 1,762 feet (537 meters) wide.

As with several of the other sinkholes on this list, Xiaozhai Tiankeng was formed after a subterranean river eroded a giant cavern out of the area’s limestone rock. When the roof of this massive cave collapsed, the sinkhole was born. Xiaozhai Tiankeng has significant tourism appeal in China, and it sometimes attracts BASE jumpers as well as people hoping to catch a glimpse of the rare fauna and flora. Exotically beautiful clouded leopards and 1,285 different types of plants have been discovered in Xiaozhai Tiankeng.

4. Zacatón – Tamaulipas, Mexico

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Zacatón, in the Mexican state of Tamaulipas, is the world’s deepest identified water-filled sinkhole, descending to an incredible 1,112 feet (339 meters). According to Dr. Marcus Gary, a hydrogeologist who has been studying the sinkhole, the massive shaft was created by volcanic activity far below the surface, which would have acidified the water – in turn gradually dissolving the rock above. Progressive collapse then led to a deeper and deeper vertical cave.

A highly unusual feature of Zacatón is the number of small islands floating freely on the surface of the water. These were created by precipitated calcium carbonate and are covered with a thick mat of grass known as “zacate” in Mexico. The islands are also shifted around the lake by the wind. Interesting microorganisms have been discovered on the walls of the sinkhole as well, and analysis confirms that there are species previously unknown to science.

3. Cave of Swallows – San Luis Potosí, Mexico

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Staying in Mexico, the Cave of Swallows, in the state of San Luis Potosí, is another limestone sinkhole – and a pretty special one at that. The sinkhole plummets to a depth of 1,214 feet (370 meters) from its highest to its lowest point. White-collared swifts, parakeets and conures (a species of green parrot) nest in the cave and fly out every morning, with the birds returning in the evening. Observing the avian creatures circling around the cave entrance and freefalling to their nests far below is a major attraction for tourists. BASE jumpers are also drawn to this huge vertical cave; these daredevils enjoy leaping off its rim before hurtling into the sprawling black void below.

2. Great Blue Hole – Lighthouse Reef, Belize

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The magnificent Great Blue Hole lies 43 miles off the coast of Belize, close to the midpoint of the Lighthouse Reef atoll. This almost perfectly circular underwater sinkhole is 407 feet (124 meters) deep and more than 984 feet (300 meters) wide. It’s a stunning geographical wonder that stands out from the surrounding turquoise water like a black eye. The Great Blue Hole was actually formed above the ocean’s surface in the last ice age. As testimony to its origins, massive stalactites spring out of the limestone cavern, and together with stalagmites and unusual columns, they make the hole seem both otherworldly and beautiful.

This submarine sinkhole is a popular destination with divers, but due to its back-sloping walls, divers should be experienced and have very good buoyancy control to plumb its depths. And the deepness of the hole also makes decompression essential. Perhaps surprisingly, inside the Great Blue Hole there is not much marine life, although lone sharks are sometimes spotted.

1. Guatemala City Sinkhole – Guatemala City, Guatemala

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Not all sinkholes were formed in remote regions millennia ago. In recent times, some have opened up right in the middle of cities, swallowing houses, people and buildings. In 2010, the sinkhole pictured above collapsed in Guatemala City, Guatemala, sending a three-story building crashing almost 100 feet beneath the ground. Fortunately, no one was inside the structure when it disappeared. A similar sinkhole arose in the same city in 2007, resulting in the deaths of three people. The Guatemalan government spent $1.5 million filling in the 2007 sinkhole, but the area was deemed unfit to inhabit.

Although the exact cause of the 2010 sinkhole is unknown, it’s possible that a cavity already existed underground for many years. The roof of the cavity might have then been weakened by water – thanks to burst pipes, poor drainage or tropical storms – until it eventually collapsed, opening up the 60-foot (18-meter) wide hole in the city.

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