Perhaps one of the most famous collections of slot canyons in the world, Zion National Park has some doozies. The Narrows is an area of canyons on the North Fork of the Virgin River and is the site of several jaw-dropping sections of slot canyons (another of which is also in this slideshow). The Narrows is considered one of the best hikes on the Colorado Plateau, and has even been named one of the best hiking adventures in the U.S. However, it's not for everyone. The canyons are formed by water erosion, and water still flows through them. This means much of the hiking is done in the river, sometimes wading waist-deep or even swimming some sections depending on the time of year. Since the walls are vertical, there is no escape should a flash flood occur, as happens during rainy periods.