The Subway, Zion National Park

Though named for mass transit, the Subway sees just 50 people a day. The oddly tubular canyon, tucked in the wilder western part of Utah's Zion National Park, is limited to hikers with permits. A segment of a seven-mile gorge, the Subway opens narrowly to the towering sandstone walls above. Its lucky few visitors must wade—at times swim—through icy pools infrequently touched by the sun. While even the easy route requires ropes, hard-core canyoneers and fans of German submarine films can take a technical one dubbed "Das Boot," where wetsuits come in handy. For those whose taste runs to Jurassic Park, dilophosaurus tracks stamp the mudstone along the canyon's creek.

Around the time the dinosaurs made their mark, some 150 million years ago, Zion's walls were laid down: A massive windblown desert blanketed the land from central Wyoming to Southern California, with its greatest depth—3,000 feet—in this corner of Utah. Compressed, uplifted, and slowly sliced by rivers, the sandstone cliffs are now among the world's highest. At the Subway, the Left Fork of North Creek has cut to the softer layer of shale beneath the sandstone, allowing the creek to widen and create the curved canyon walls.

While primordial forces are showcased at Zion, the park also tells stories of 8,000 years of human history. Ancient yucca-fiber sandals—the earliest residents' footwear—were found in local caves. Art of the Anasazi, who disappeared around a.d. 1300 after centuries in the region, still decorates the rock. More recent canyon dwellers left their legacy with names: The main canyon was christened "Zion," from the Hebrew word meaning a place of refuge, by 1860s Mormon pioneers who had escaped persecution back East. Not far from the Subway, Tabernacle Dome rises, while farther down the creek, Archangel Cascades fall.

--Elisa Freeling