Buckskin Gulch, Utah
The longest and deepest slot canyon in the Southwest U.S. (and quite possibly the longest in the world), Buckskin Gulch stretches for over 13 miles with the cliffs on either side reaching as high as 500 feet. It is one of the main tributaries of the Paria River, which flows only intermittently. Because of the depth of the canyon, it doesn't fill with light the way Antelope Canyon does. But the length and diversity of terrain draw in experienced hikers. A permit is required to hike into the area, and the permits usually sell out months in advance, so if you want to hike this slot canyon, start planning way ahead of time. Also be aware this is a challenging hike and there is a risk of flash floods during certain seasons — as there is with most slot canyons.