THE GREAT BLUE HOLE 
From the air, the Great Blue Hole of Belize resembles an otherworldly maw, intent on drinking down the surrounding Lighthouse Reef Atoll. In reality, the 1,000-foot (305-meter) wide hole is simply a sinkhole in the ocean. Geologists believe that an underlying cave system collapsed under increased pressure some 10,000 years ago due to rising sea levels.
The dark hole descends 412 feet (126 meters), terminating in lightless depths where a lack of oxygen prevents most forms of life from thriving. Divers rarely plunge these depths, however, as most are content to explore the stalactite-rich caverns accessible from depths of some 130 feet (40 meters) below the surface.
From hidden depths to towering vistas, these are just 10 of Earth's countless geologic wonders -- each more than a rival for anything dreamed up for fiction and fantasy. We live on a spectacular planet; you just have to open your eyes to it.
Explore the links on the next page to learn even more about Earth's amazing geology.
SUBTERRANEAN SPLENDOR
The hill country of Kentucky's Green River Valley certainly has its charms, but beneath its gentle woodlands there is an underworld. More than 390 miles (628 kilometers) worth of caves worm through the rocky depths, making Mammoth Cave the largest known cave system in the world.
For more than 10 million years, waters from the Green River have cut through the soft limestone, riddling it with all manner of cave formations. A visitor may pass through a lengthy passageway and
into a vast cathedral. Vertical shafts descend into darkness while stalagmites, stalactites and bizarre crystal formations speak to the immensity of geologic time.
Yet Mammoth Cave is not just a place of minerals, stones and tourists. It also boasts an impressive ecosystem of cave flora and fauna, encompassing more than 130 documented species.
DEVILS TOWER
When an enormous column of rock towers 1,267 feet (386 meters) above the surrounding landscape, people take notice. That's why Teddy Roosevelt declared Devils Tower America's first national monument, and Steve Spielberg decided to land a UFO on top of it in "Close Encounters of the Third Kind. Long before either man thrust the natural landmark into the spotlight, more than 20 Native-American tribes held the site sacred -- including the Lakota people who dubbed it "Bear Lodge."
While its exact origins are unclear, many geologists believe the enormous column of igneous rock is an intrusion: a column of molten rock pushed up from the inner Earth through sedimentary rock layers. It's unclear whether the intrusion cooled before or after it breached the surface, but the vertical furrows indicate that cooling and contraction took place.
Geologists suspect the northeastern Wyoming tower formed more than 50 million years ago and remained buried beneath the ground up until roughly 2 million years ago [source: SERC]. Today, Devils Tower is a popular tourist destination, and licensed climbers can even scale the monolith for an unforgettable view.
THE FAIRY CHIMNEYS
You want a fantastic real-world locale? It's hard to improve upon the fairy chimneys of Cappadocia in central Turkey. Here, tall spires of stone dot the landscape like some manner of bizarre growth. What's more, early Christians carved countless storerooms, stables and domiciles into the fairy towers from the 4th to the 11th century.
To understand the formation of these stone spires (also known ashoodoos), look at the accompanying photograph. The fairy chimneys you see here were once part of a massive slab of earth covered in a layer of hardened volcanic ash called tuff. Over time, the erosive forces of wind and water wore away much of the underlying soft material, leaving only slender towers with caps of tuff.
The early Christians went so far as to carve whole monasteries and underground cities out of the stone in Cappadocia. Today, many such chambers are still in use -- some as guesthouses for visiting tourists.
THE FOREST OF KNIVES
Madagascar is truly a lost world. Cut off from the rest of the world, the island's lemur population thrived (they don't exist anywhere else on the planet, except in captivity), and a host of unique life forms evolved in relative isolation. Yet Madagascar's geology also stands apart from the rest of the world's -- especially the region known as Tsingy de Bemaraha.
Here, visitors encounter a forest of upturned limestone daggers. This painful-looking landscape, also known as karst topography, results from long-term dissolution of soluble limestone bedrock. Formerly a massive slab of rock, rainwater has whittled it down into multiple, individual towers of stone. The Tsingy de Bemaraha National Park protects a 600-square-mile (1,554-square-kilometers) region of stone and vegetation.
The inhospitable nature of the tisngy serves to protect a host of creatures, many of which avoided discovery by humans until the 21st century.
THE CRYSTAL CAVERNS
Nearly 1,000 feet (305 meters) beneath Mexico's Naica silver mine you'll find a chamber of unearthly wonder. Here in Cueva de los Cristales (the Cave of Crystals), 36-foot (11-meter) obelisks of solid crystal lay heaped about like fallen pillars in a dilapidated temple.
This subterranean forest of wonders boasts the largest known gypsums (soft minerals made of hydrate calcium sulfate) on Earth. For roughly half a million years, the hidden chamber was nothing short of a crystal incubator. For starters, nearby magma deposits heat the cavern to temperatures of up to 112 degrees Fahrenheit (44 degrees Celsius). And to top things off, the entire space was flooded with mineral-rich waters up until very recently.
The chamber was discovered in 2000, after mining operations pumped it dry. Today, only a few visitors risk heatstroke to witness the crystals' beauty firsthand.
THE REFLECTING DESERT
Earth's vast, barren expanses are often as awe-inspiring as its highest peaks and deepest valleys. Just consider the Bolivian Uyuni Salt Flats, or Salar de Uyuni, a 4,000-square-mile (10,360-square-kilometer) plane of what appear to be hexagonal tiles. This extraordinary high-altitude landscape stretches among the snow-peaked Andean mountains, and if you happen to visit during the rainy season, you're in for quite a sight.
When the rains sweep down onto the Uyuni Salt Flats, the entire expanse becomes an immense reflecting pool. The water on the salt flats never reaches a depth of more than 6 inches (15 centimeters), so it offers visitors the unique sensation of walking on the surface of a mirror -- all amid a desolate silence.
The unique landmark is actually the remnant of a prehistoric lake and currently ranks as the largest salt flat in the world.
FOG-SHROUDED PEAKS
You'll find no shortage of breathtaking vistas on the Greek peninsula, but the Meteora rock formations truly take the cake. These massive sandstone fingers seem to emerge as much from a dream as from the plains of Thessaly. Towering as high as 2,044 feet (623 meters) above lush landscape below, the steep peaks of Meteora are a perfect setting for a secluded monastery.
Monks and nuns have called Meteora's peaks and caverns home for centuries. Hermits scaled the daunting peaks as early as the 10th century and, according to legend, St. Athanasios Meteorites rode an eagle to the top in the 1300s to found Great Meteoron, the largest of the region's six secluded monasteries.
The monasteries remain active to this day, though some peaks remain rather isolated destinations. Up until 1925, visitors could only reach Ayia Triada (aka Hagia Triada) monastery via rope ladders and baskets. Today, it boasts a 140-step staircase hewn into the rock.
THE CRACK OF SILFRA
Conflict is the meat of great storytelling. You might prefer such tropes as man-versus-nature or man-versus-blue-aliens, but the best geological drama often unfolds when tectonic plates duke it out, especially continental plates. Travel to Iceland, however, and you'll find a most curious occurrence on the boundaries of the North American and European plates.
Adjacent to Lake Thingvalla, you'll find Silfra Crack. Filled with crystal-clear, glacial meltwater, this narrow slit plunges 66 feet (20 meters) into the Earth. It makes for a rather chilly descent, but sight-seeking divers make the pilgrimage each year to dive between the continents. Experienced cave divers can explore depths of more than 148 feet (45 meters) by swimming into the Silfra cave system.
Visitors frequently describe the Silfra diving experience as one of floating weightlessly through space. The glacial waters filter through miles of volcanic rock before emptying into the crack.
ULURU THE MONOLITH
In "Avatar," a noble, indigenous people fight to protect their sacred landmarks against an invading culture. If you're pining for that sort of drama, then look no further than Australia's Uluru-Kata Tjuta National Park. Here you'll find mighty Uluru, one of the largest geologic monoliths in the world.
Dubbed "Ayers Rock" by Europeans, the 5.8-mile (9.4-kilometer) wide slab of arkose (a type of sandstone) resonates with sacred significance for the Anangu people. Aboriginal paintings pepper its base, as well as caves and waterholes held sacrosanct in the spiritual tradition of Tjukuritja. While the Anangu have visited the site for roughly 22,000 years, they only regained legal ownership of the land in 1985 after a century of European rule.
Uluru is the visible tip of a much larger rock slab that extends deep into the Earth. In ages past, this tip was underground as well, but hundreds of millions of years of erosion have reduced the surrounding landscape. Uluru gets its red complexion from clay and rusted iron minerals within the sandstone. At dusk and dawn, the monolith takes on even darker, crimson hues.
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