Sixty million years ago, the Rocky Mountains and the entire Colorado Plateau, which the Grand Canyon is part of, rose up from tectonic activity. After the top layers eroded away, the Colorado River began to cut its way through the ancient rock, leaving the stunning canyon we see today.

The Hopi have always placed great spiritual significance on the Grand Canyon. They believe that upon death, a person passes westward through a "place of emergence", located upstream from the confluence of the Colorado and Little Colorado rivers in the canyon, on his or her journey to the afterlife.

At its deepest point, the Grand Canyon is approximately 6,000 feet deep--that's more than a mile. But it's not the deepest canyon in the world. That honor goes to the Tsangpo Canyon in Tibet, which reaches a maximum depth of 19,714 feet--almost 4 miles!

Hidden within the Grand Canyon are an estimated 1,000 caves, but only 335 have been recorded, and only one (the Cave of the Domes on Horseshoe Mesa) is open to the public.

While the Grand Canyon might seem like the perfect spot to go hunting for dinosaur bones, none have ever been found there. The rock that makes up the canyon walls is vastly more ancient than the dinosaurs--about a billion years more ancient, in some cases--although the canyon itself probably didn't form until after the dinosaurs were long gone. There are lots of other fossils, however, ranging from ancient marine creatures dating back 1.2 billion years to fairly recent land mammals that left their remains in canyon caves about 10,000 years ago.

The word "Anasazi" is Navajo for "Ancient Ones". Toward the end of the 13th century, some cataclysmic event forced the Anasazi to flee their cliff houses and move south and east toward the Rio Grande and the Little Colorado River. Just what happened is the greatest puzzle facing archaeologists who study the ancient culture. Today's Pueblo Indians have oral histories about their peoples' migration, but the details of these stories remain closely guarded secrets.

The capital of the Havasupai Indian Reservation, Supai is 8 miles (13 km) from the nearest road. It is the only place in the United States where mail is still carried out by mules.

Based on local emergency room visits, the rock squirrel is actually the most dangerous animal of all the park's inhabitants. Thousands of these very large ground squirrels (17 inches to 21 inches long) hang out in the national park, and Park Rangers have described them as both "unforgiving" and "absolutely ferocious".

At its widest point the Grand Canyon stretches 18 miles (29 kilometers) across. At its narrowest point, at Marble Canyon, it is only 0.34 miles (548 meters) across.

The Grand Canyon is home to six species of rattlesnakes: the black-tailed rattlesnake, Grand Canyon pink rattlesnake, Great Basin rattlesnake, Hopi Rattlesnake, Mojave Rattlesnake, and speckled rattlesnake. These creatures control rodent populations in the Canyon, helping to prevent the spread of disease and the overgrazing of fruiting plants.