Erg Chebbi: The only Saharan erg in Morocco. Ergs are large and relatively flat areas of the desert that are covered with wind-blown sand, forming dunes; Erg Chebbi’s dunes can grow to nearly 500 feet high.
Niagara Falls: Known for their massive beauty that lures people from all around the world (especially newlyweds!), Niagara Falls separates Ontario, Canada, from New York state. Horseshoe Falls, on the Canadian side, is about 2,600 feet wide, while the American Falls is 1,060 feet wide.
Ayers Rock: A natural icon of Australia, Ayers Rock (also known as Uluru) is one of the largest monoliths in the world. The sandstone formation, which appears to change colors with different amounts of light, was once an island in a large sea. It rises 1,142 feet from the desert floor and has a circumference of about 6 miles; much more of its mass remains underground.
Okavango Delta: The world’s largest inland delta supports one of the highest concentrations of wildlife in Africa, including large numbers of crocodiles, wild dogs, elephants and hippos. Seasonal flooding expands the delta’s area from fewer than 3,500 square miles to more than 6,500 square miles; about 95 percent of its water evaporates in the Kalahari Desert.
Amazon River: The massive Amazon River dumps between 9 million and 32 million gallons of water per second into the Atlantic Ocean. It has more than 1,000 known tributaries, and 17 of those are at least 1,000 miles long. No bridges cross the river, whose main stream is 50 miles wide.
Franz Josef Glacier: One of the world’s steepest and fastest flowing glaciers, the Franz Josef plunges more than 8,000 feet in six miles. Located in the Southern Alps of New Zealand’s South Island, it is also unique in its descent through a temperate rain forest.
Mount Kilimanjaro: The tallest free-standing mountain in the world and the highest mountain in Africa is a dormant volcano with two peaks. Kibo, the higher peak, looms over northeastern Tanzania at 19,341 feet above sea level; the summit of Mawensi is seven miles away and rises to 16,893 feet.
Milford Sound: The centerpiece of New Zealand’s Fiordland National Park on the country’s South Island, Milford Sound stretches nine miles inland from the Tasman Sea with sheer rock faces that rise 3,900 feet or more on each side. Residents of the sound include seals, penguins and dolphins.
Redwood National and State Parks: The coast redwood, the world’s tallest tree and one of its most massive, is protected in these four parks, which comprise 112,613 acres along the Pacific Ocean in northwestern California. One of the world’s tallest trees, 378.1 feet, is found here in the virgin forests of ancient redwoods, along with ponderosa pine, Douglas fir and Sitka spruce.
Great Blue Hole: This underwater sinkhole, a system of limestone caves that was flooded after the last ice age, lies 60 miles off the coast of Belize in Lighthouse Reef. The almost perfectly circular hole is more than 1,000 feet across and 400 feet deep, with otherworldly stalactites and limestone formations along its walls.
Flaming Cliffs: The rock formation in Mongolia’s Gobi Desert is best known as the site where dinosaur eggs were first discovered. The site is named for the orange glow that bathes the rocks.
The Everglades: Known as the "River of Grass," this Florida park is the largest subtropical wilderness in the U.S. Some 15 threatened and protected species live within the Everglades, including the American crocodile, Florida panther and West Indian manatee.
The Camargue: France’s brine ponds are one of the few habitats for the greater flamingo in Europe (and for some of the most voracious mosquitoes in France), and are also home to wild bulls and white horses. At 360 square miles, this region of marshes, lagoons and farms in southern France is Western Europe’s largest river delta.
Mount Everest: The highest mountain on Earth (29,035 feet), and perhaps the peak most shrouded in mountain-climbing lore, Everest is part of the Himalaya range, located on the border between Nepal and Tibet.