The Desert Hidden Between These Mountains, Colorado

At the foot of the Rocky Mountains, in southern Colorado, there's a field of sand dunes that shouldn't seem possible. Snow-capped peaks rise more than 13,000 feet behind it, and in front of them, piles of sand stand taller than the Washington Monument. These are the tallest sand dunes in North America. The dunefield covers about 30 square miles on the eastern edge of the San Luis Valley, a broad, high basin between the San Juan Mountains to the west and the Sangre de Cristo Mountains to the east. The valley floor itself sits at around 7,500 feet. The sand came from the mountains. Over hundreds of thousands of years, rivers and streams carried sediment down from both ranges, and ancient lakes filled the valley. When those lakes dried up, they left behind vast deposits of sand. The dunes began forming within the last 440,000 years or so, and much of their sand is volcanic rock fragments worn down from the San Juans. Then the wind went to work. The valley's prevailing winds blow from the southwest, carrying sand across the valley floor until it hits the wall of the Sangre de Cristo Mountains. But storm winds blow back the other way, down through the mountain passes. When wind pushes from one direction, dunes migrate and stay low. When it pushes from several directions, the sand piles up on itself and grows. That tug-of-war is what makes these dunes so tall. Water finishes the cycle. Medano Creek catches sand that blows toward the mountains and carries it back around to the edge of the dunefield, where the wind picks it up again. Every grain gets used over and over. The tallest dunes here trade the record back and forth. According to the National Park Service's official map, Star Dune stands about 736 feet and High Dune about 688 feet. The landscape is strange enough that planetary scientists study it as a stand-in for the surface of Mars. At least six species of insects live here and nowhere else on Earth, including the Great Sand Dunes tiger beetle. And because the park is so remote, it's an International Dark Sky Park. The dunes were protected as a national monument long before becoming a national park on September 13, 2004. Today, visitors can climb, sled and sandboard anywhere they like. The mountains still feed the sand, the wind still builds the dunes, and the creek still carries the sand back to start again.