The Grand Canyon is a steep-sided canyon carved by the Colorado River in Arizona, United States. It is 446 km long, 29 km wide and reaches a depth over 1800 meters. The Colorado River established its course trough 5 - 6 million years ago. Since then, it has been gradually deepening and expanding. The erosion exposes one of the most complete geologic columns on the planet. For thousands of years, the area has been continuously inhabited by Native Americans, who built settlements within the canyon and its many caves. The Grand Canyon area has some of the cleanest air in the United States. The canyon's ecosystem was permanently changed after the construction of the Glen Canyon Dam in 1963. Grand Canyon National Park is one of the oldest national parks in the US. It was founded in 1919 and iIt is one of the world's leading natural attractions, attracting approximately five million visitors a year.