Grand Canyon National Park, located in northwestern Arizona, was declared the 15th national park in the United States. The park's star attraction is the Grand Canyon, a stunning gorge of the Colorado River, which is often considered one of the Wonders of the World. Covering an area of 1,217,262 acres (1,901.972 sq mi; 4,926.08 km2) in Coconino and Mohave counties, the park welcomed over six million recreational visitors in 2017, making it the second most visited national park in America after Great Smoky Mountains National Park. Grand Canyon National Park was listed as a World Heritage Site by UNESCO in 1979 and celebrated its 100th anniversary on February 26, 2019.