Landscapes collide in a shifting sea of sand at Great Sand Dunes National Park, making you wonder whether a spaceship has whisked you to another planet. The 30-sq-mile dune park - the tallest sand peak rises 700ft above the valley floor - is squeezed between the jagged 14,000ft peaks of the Sangre de Cristo and San Juan Mountains and flat, arid scrub-brush of the San Luis Valley.