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Salar de Uyuni, Altiplano, Bolivia (© Theo Allofs/Corbis)

Salar de Uyuni is a magical place: When covered by water, the world’s largest salt flat becomes a mirror, and anyone walking across it appears to be walking on clouds. The salt crust, which covers 4,086 square miles in southwestern Bolivia at 11,995 feet above sea level, is nearly flat, which makes it ideal for calibrating the altimeters of satellites.


Giants Causeway, County Antrim, Northern Ireland (© Spila Riccardo/SIME/4Corners Images)

The Giants Causeway in Northern Ireland. Volcanic activity 60 million years ago created these rugged, symmetrical rock formations, seemingly steppingstones that lead into mists and legend.


Ancient 'geoglyph' wheels, Mideast desert (© David Kennedy/courtesy APAAME)

Archaeologists have discovered thousands of these “floor drawings” of stones in Syria, Jordan and Saudi Arabia. The wheels measure from 82 feet to 230 feet across and could be at least 2,000 years old; other stone structures are far older.


Sliding rock at Racetrack Playa, Death Valley, Calif. (© Exactostock/SuperStock)

Even NASA cannot explain it. It’s best to gaze in wonder at the sliding rocks on this dry lake bed in Death Valley National Park. Racetrack Playa is almost completely flat, 2.5 miles from north to south and 1.25 miles from east to west, and covered with cracked mud. The rocks, some weighing hundreds of pounds, slide across the sediment, leaving furrows in their wakes, but no one has actually witnessed it.


Spotted Lake, British Columbia, Canada (© All Canada Photos/Alamy)

Many minerals are found in high concentrations in Spotted Lake; that causes the phenomenon that gives the lake its name.  Spots form during summer when much of the water evaporates, leaving the minerals, which harden and form walkways among the spots. The water’s color is determined by the unique combination of minerals. The site, near Osoyoos in British Columbia, is owned by the First Nations and is not open to the public.


Lime-sinter terraces at sunset, Pamukkale, Anatolia, Turkey (© Mauritius/SuperStock)

Cotton Castle, Pamukkale’s translated name, is a wildly popular tourist site. Seventeen hot-water springs in the area spill out water in temperatures ranging from 95 degrees to 212 degrees, which contains a high concentration of calcium bicarbonate. The water flows off a cliff, cools and hardens into calcium deposits that form terraces, as white as cotton and bright enough to be easily seen from the town of Denizli, which is on the opposite side of the valley, 12 miles away.


Split Apple Rock, Abel Tasman National Park, Southern Island, New Zealand (© Uli Wiesmeier/Corbis)

Split Apple Rock.  The formation is a big hit with visitors to Abel Tasman National Park on the South Island of New Zealand. It is just one of the highlights in the park, which was founded in 1942, 300 years after Dutch seafarer Abel Tasman became the first European to visit New Zealand.


Aerial view of Blue Hole, Belize (© Simeone Giovanni/SIME/4Corners Images)

In 1971, Jacques Cousteau boldly sailed Calypso to the Great Blue Hole, investigated and declared it one of the 10 best diving sites in the world. It’s a large underwater sinkhole near the center of Lighthouse Reef, about 62 miles from Belize City.  The circular hole is nearly 1,000 feet across and 410 feet deep, boasting underwater caves, fantastic coral formations and many species of tropical fish darting through the clear water.

The Stone Forest, Yunnan Province, China (© José Fuste Raga/Corbis)

 “If you have visited Kunming without seeing the Stone Forest, you have wasted your time.” The Shilin Stone Forest covers 96,000 acres with large and small stands of stone “trees.” They actually are karst formations that stand on the earth like stalagmites and looking like petrified trees. Believed to be more than 270 million years old, the stone trees emerged as limestone eroded.


Mud volcanoes, Caspian coastline, Republic of Azerbaijan (© Wolfgang Kaehler/SuperStock)
Take what you know about volcanoes and imagine mud volcanoes. Rather than hot lava, steam, ash and rocks, mud volcano eruptions involve cold mud, water and gas. Nearly 400 mud volcanoes, more than half of the world total, are found in Azerbaijan and the Caspian Sea. The most common have several small cones or vents and average about 13 feet high.


Moaning Cavern, Vallecito, Calif. (© Dave Bunnell)

The sounds echoing through the cavern might be unnervingly like a human moan. But the sound is created by water dripping into holes in the bottom of the formation, which causes a drumming sound that echoes off the walls and is carried out of the Moaning Cavern‘s natural entrance by the wind. Gold miners came upon this cavern in 1851 (it is near Angels Camp), but it has been known far longer; some of the oldest human remains known in the Americas were found here.

Moeraki Boulders, Otago, South Island, New Zealand (© Kevin Schafer/Corbis)

The Moeraki Boulders are a big attraction, found on Koekohe Beach near Moeraki on New Zealand’s coast. The huge, gray, spherical stones formed in sediment on the sea floor 60 million years ago and were revealed by shoreline erosion. Or, if you take the local Maori perspective, they are the remains of calabashes (gourds), kumaras (sweet potatoes) and eel baskets that washed ashore when the legendary canoe Araiteuru was wrecked. Either way, the boulders, some of which stand alone and some in clusters, can weigh several tons and measure 10 feet across.