Nankoweap Trail, Grand Canyon National Park, Arizona

Dizzying heights and scorching temperatures are the primary hazards on the Grand Canyon's well-trafficked trails. And the Nankoweap Trail, which the NPS classifies as one of the most difficult in the park, is no exception. The views along the way—panoramas of kaleidoscopic canyons—are unparalleled, but so are the dangers. The Nankoweap Trail has the largest rim-to-river drop in Grand Canyon National Park, more than 5,000 feet above the Colorado River. According to the NPS, "The trail is often only one footprint wide, loose and gravelly, with a 10 to 150 [foot] drop-off. This trail is not recommended for people with a fear of heights."