Sure, it's popular, but it's popular for a reason. With its eponymous four passes (West Maroon, Frigid Air, Trail Rider, and Buckskin) totaling roughly 10,000 feet of elevation gain over 28 miles in the Maroon Bells-Snowmass Wilderness, the Four Pass Loop requires three days, including one grueling stretch that covers two passes. The workout pays off, however, in the form of superlative views dominated by red rock, green leaves, and blue sky, not to mention the crystalline alpine lakes teeming with trout.