At one mile deep and 277 miles long, the world's most famous gorge offers a glorious vista of colourful rock formations that never fails to surprise the first-time visitor with its grandeur. Although there are wider and deeper canyons elsewhere in the world, this is the most visually spectacular. Carved out by the Colorado River below - itself a big draw for whitewater rafting - the Grand Canyon attracts five million visitors a year. You can avoid the worst of the crowds by visiting the North Rim in spring/summer, the South Rim in winter - when the views are crisp (and the parking easy). And check out the remote West Rim's new-ish SkyWalk, a glass-bottomed bridge built in 2007 that extends out over the canyon, for a scarily crystal clear view ($30 or less when bought as part of a tour).