Our planet boasts a wide variety of magnificent vistas and awe-inspiring scenes, but the following seven attractions are often touted as the most amazing. Spread across five continents, these "seven wonders of the natural world" attract and delight millions of visitors each year. While other lists celebrate the greatest achievements of engineering and architecture, what is most remarkable about these natural wonders is that none of them was either built or improved upon by human hands.

Grand Canyon, Arizona, United States

Grand Canyon (Photo Credit: AETN)

The Grand Canyon, located in northwestern Arizona, is the product of millions of years of excavation by the mighty Colorado River. The chasm is exceptionally deep-it drops more than a mile into the earth-and is 15 miles across at its widest point. Each year, the 277-mile-long canyon attracts more than 5 million visitors, who are treated to sweeping views of the steep gorges' colorful walls and towering buttes. The canyon is home to more than 1,500 plant species and over 500 animal species, many of them endangered or unique to the area, and its well-preserved walls tell the story of 2 billion years of Earth's history. In 1540, members of an expedition sent by Spanish explorer Francisco Vásquez de Coronado became the first Europeans to see the canyon, though because of its remoteness the area was not further explored until 300 years later. American geologist John Wesley Powell, who popularized the term "Grand Canyon" in the 1870s, became the first person to journey the entire length of the gorge in 1869. The harrowing voyage was made in four rowboats. In January 1908, U.S. President Theodore Roosevelt designated more than 800,000 acres of the Grand Canyon a national monument; it became a national park under Woodrow Wilson in 1919.

Great Barrier Reef, Australia

Great Barrier Reef (photo credit: Corbis)

The 1,200-mile-long Great Barrier Reef, located off the northeastern coast of Queensland, Australia, is actually a chain of some 2,800 smaller reefs. The world's largest coral reef system and the largest marine preserve in the world, the reef is widely regarded as one of Earth's most important--and beautiful--natural sites. The more than 2 million people who visit the reef annually are treated to an ever-shifting kaleidoscope of multi-colored coral, abundant plant life and vividly painted tropical fish. The reefs that make up the Great Barrier Reef were formed when the calcium-containing exoskeletons of dead coral animals, among other marine species, built up year after year on a shallow area of sea floor. Living corals then grew on top of the remains, building up the reefs at the rate of just one to 100 centimeters per year. In part because of this slow growth, the reefs are extremely fragile and require strict protection from most human interference to survive. In total, more than 400 species of coral, 1,500 species of fish, 4,000 species of mollusks and large numbers of turtles, manatees, manta rays and sharks make their home among the diverse ecosystems of the Great Barrier Reef. Although it is the best-protected coral reef system in the world, climate change presents a serious threat to its continued health.

Harbor of Rio de Janeiro, Brazil

Rio de Janeiro’s Harbor (photo credit: Corbis)

Rio de Janeiro’s Harbor at Night (photo credit: Peter Guttman)

One of the many reasons Brazil's Rio de Janeiro is nicknamed a cidade maravilhosa or the "marvelous city" is its spectacular natural harbor, which is one of the largest in the world. Surrounded by crescent-shaped white sand beaches, lush rainforest and the 1,325-foot Sugar Loaf Mountain, or Pão de Açúcar, the harbor is renowned for its magnificent beauty. Also overlooking the harbor is nearby Corcovado Mountain, nearly twice as tall as Sugar Loaf, and its 120-foot soapstone statue of Jesus with his arms outstretched. Known as Christ the Redeemer, the statue was built to commemorate the 1922 centennial of Brazilian independence. The first Europeans to see Rio's stunning harbor were Portuguese explorers, who sailed into Guanabara Bay from the Atlantic Ocean in the early 16th century. At the time, the area was home to Tupí Indians. In an effort to keep away French traders, the Portuguese finally founded the city that would become Rio, originally called São Sebastião do Rio de Janeiro, in 1565. The area was later involved in the production and shipping of sugar cane, and in the 18th century, the harbor was as an important port in the export of gold and diamonds from Brazil's mines. The harbor is deep enough to accommodate even today's largest ships, and most of Brazil's imports and exports still come through Rio's port.

Mount Everest

Mount Everest (photo credit: Peter Guttman)

At 29,035 feet above sea level, Mount Everest, known as Chomolungma or "goddess-mother" in the Tibetan language, is the tallest mountain in the world. Located at the border of Tibet and Nepal, Everest is part of the Himalayan mountain range in south central Asia. The summit of the mountain, which reaches two-thirds of the way through Earth's atmosphere, is an extremely forbidding destination for human beings. Oxygen levels there are very low, temperatures are very cold and the weather is unpredictable and dangerous. Also, Sherpas from the Himalayan region of Nepal long believed Everest to be a holy place that served as a home to the gods, and climbing its slopes was thus forbidden. Sherpas did however begin to venture into the peaks to assist European mountaineers and explorers in the 1920s. In May 1953, Edmund Hillary of New Zealand and Tenzing Norgay, a Nepalese Sherpa, became the first men to reach Everest's summit. Today, in addition to the ever increasing number of people attempting to scale the mountain, Everest's base camp has become a popular tourist attraction. Unfortunately, like many of the world's great natural wonders, Mount Everest is being threatened by climate change, which has already caused as much as a 30 percent decrease in ice and snow on other Himalayan peaks.

Aurora Borealis

The Northern Lights-- Aurora Borealis (photo credit: Corbis)

The Aurora Borealis, also called the Northern Lights, is a spectacular display of dancing light that appears in the northern nighttime sky. The name aurora is the Latin word for dawn and refers to the Roman goddess of the dawn, while borealis comes from the name of the Greek god of the north wind, boreas. Often referred to as "nature's light show," the Aurora Borealis went unexplained for years, and both inspired and frightened early peoples. Today, it is known that the phenomenon occurs when an excess of charged particles from the Sun, often caused by solar flares, streams into Earth's atmosphere. The particles are sent toward Earth's poles, where they collide with gas particles that then emit light, causing a glow high in the atmosphere. The Northern Lights can take on a variety of forms including arcs, vertical bands, low-hanging clouds, fans and flames. The shapes can be relatively still or in constant motion and come in a multitude of colors, including green, white, yellow and red. The best place to see the Aurora Borealis is at about 20 degrees from the North Pole, though it is visible across the northern part of North America and Eastern Europe, and has sometimes been seen as far south as Key West, Florida. A similar occurrence that takes place over the southern sky is known as the Aurora Australis.

Parícutin volcano, Michoacán, Mexico

Paricutin Volcano (photo credit: Corbis)

In February 1943, in the central Mexican state of Michoacán, shocked villagers reported something no modern scientist had ever witnessed: the birth of a volcano. Seemingly out of nowhere, what is now known as the Parícutin volcano began to erupt out of a cornfield in the village of Parícutin, about 200 miles west of Mexico City. According to local legend, a farmer was out tending his fields when the ground around him began to boil and steam began to gush out of large cracks that opened in the Earth. The farmer first tried to plug the holes, but then fled the area when he realized the task was impossible. Many of the local people did not understand what was happening and believed that the end of the world was upon them. Despite initial reluctance, everyone in the neighboring towns was eventually convinced to evacuate. This proved providential, as over the course of the next two years, the volcano grew to over 1,000 feet and buried most of Parícutin and the neighboring town of San Juan Parangaricutiro under lava. After nine years of eruptions, the volcano covered a full 10 square miles and reached 1,390 feet into the sky. The volcano has remained inactive since 1953. Today, visitors to this natural wonder can take a 14-mile round-trip hike to the top of the volcano's cinder cone for sweeping views of the surrounding countryside.

Victoria Falls, Zimbabwe

Victoria Falls (photo credit: Corbis)

Victoria Falls, one of the largest and most spectacular waterfalls in the world, is located at the border of Zambia and Zimbabwe in southern Africa. There, the mile-wide Zambezi River drops 400 feet over a series of basalt gorges, sending spray as high as 1,000 feet into the air during the spring wet season. Depending on river levels, anywhere from 5 million to 132 million gallons of water can rush over the falls in a single minute. All that water makes a lot of noise, and many Southern Africans refer to the falls as Mosi-oa-Tunya or "the smoke that thunders" in reference to its deafening roar. The falls are visible as far as 40 miles away. In November 1855, Dr. David Livingstone, the famed Scottish explorer and missionary, became the first European to see the falls. It was Livingstone that christened them "Victoria Falls" after the British monarch Queen Victoria. Today, the falls make up part of Zambia's Mosi-oa-Tunya National Park, home to the country's last remaining rhino. Some of the best white-water rafting in the world lies at the foot of the falls, and the truly adventurous can bungee-jump off a nearby bridge.