Below are some of the world's most extraordinary natural wonders. Guided tours provide easy access to these special places. Click the name of any attraction below to see a photo.
Carved by the Colorado River, Arizona's Grand Canyon measures 277 miles long, four to 18 miles wide and one mile deep. It offers dramatic, beautifully colored landscapes and is one of the most studied geological areas in the world. Hiking, mule rides and white-water rafting are popular ways to explore the canyon. Click here for tours that visit the Grand Canyon.
Redwood National Forest in Northern California is home to the world's tallest trees. A redwood, which can be hundreds of years old, can reach more than 200 feet tall and up to 20 feet in diameter. Click here for trips that include the redwoods.
The Giant's Causeway on Northern Ireland's coast is an unusual sight of about 40,000 interlocking, hexagonal basalt columns that form steps down to the sea. The UNESCO World Heritage Site is the result of a volcanic eruption, but legend says the natural wonder was built by a giant named Finn McCool. Click here for tours that stop at the Giant's Causeway.
Formed by erosion from the Atlantic Ocean, the harbor of Rio de Janeiro in Brazil is the largest natural bay in the world based on volume of water. Also known as Guanabara Bay, the harbor is surrounded by monolith granite mountains, including Sugar Loaf Mountain, the Tijuca hills and Corcovado Peak (home to the iconic Christ the Redeemer statue). The bay includes over 130 islands. Click here for trips that visit Rio de Janeiro.
Fed by Zambia's Zambezi River, Victoria Falls is the world's largest waterfall, based on height and width of the water sheet during the rainy period. The mist created by the falls as the thundering curtain of water plummets into the Zambezi Gorge can rise 100 feet in the air, explaining the name given by the Kololo tribe in the 1800s: Mosi-oa-Tunya -- "the smoke that thunders." Click here for vacations that take you to Victoria Falls.
One of the most incredible places on Earth is Tanzania's Ngorongoro Crater. Created when the upper two-thirds of a towering volcano collapsed into its base, the Ngorongoro Crater is the world's largest unbroken caldera. Often called "Africa's Garden of Eden," it encompasses more than 100 square miles of desert, grasslands, forests, lakes and streams, and it contains more than 25,000 large animals and the greatest concentration of mammalian predators in the world. Click here for safaris that spend time at Ngorongoro Crater.
Feel the spray from impressive Iguazu Falls, which spills into Brazil and Argentina. This horseshoe-shaped site measures 1.7 miles wide, and up to 275 separate cascades have been counted, the tallest plunging more than 260 feet. The surrounding subtropical rainforest is home to jaguars, howler monkeys and giant anteaters. Many itineraries view the falls from both the Argentine and Brazilian sides. Click here for tours that highlight Iguazu Falls.
Seek out the elusive northern lights, or aurora borealis, in Alaska, Canada, Greenland, Iceland, Norway or Finland. This natural phenomenon is caused by electrically charged solar particles entering the Earth's atmosphere, resulting in great swaths of green, blue and red lights sweeping across the night sky. They are most visible from September through March. Click here for departures that seek out the northern lights.