The Grand Canyon is a mile-deep gorge in northern Arizona. Scientists estimate the canyon may have formed 5 to 6 million years ago when the Colorado River began to cut a channel through layers of rock. Humans have inhabited the area in and around the canyon since the last Ice Age. The first Europeans to reach the Grand Canyon were Spanish explorers in the 1540s. President Benjamin Harris first protected the Grand Canyon in 1893 as a forest reserve, and it became an official United States National Park in 1919.