The Great Sand Dunes National Park, in Colorado, United States. (scanned from original 35mm chrome)
These sand dunes are huge; they are literally mountains of sand. The two “specs” hardly visible in the highlighted area just left of center is actually two people trekking across the range. I trekked it myself years ago. If you know how hard it is to walk on sand at the beach, its more difficult uphill. The more you walk the more it seems you aren’t getting anywhere.
Its an incredible journey and people get lost after nightfall quite easily, especially as the landscape and imagery continues to change throughout the day between the wind and shadow play. Some day I would love to do a time lapse of this area.