These sand dunes are the largest in North America. About 440,000 years ago, a lake called Lake Alamosa evaporated revealing a sand sheet. The winds coming over the passes and an opposing wind current during storms cause these dunes to grow vertically. The largest dune as of now is 750 feet tall and is still growing. Sand is brought in each year from top of the mountains through streams that flow to the valley floor, these streams are seasonal and dry up during the summer.
When you visit this incredible site, don't forget to rent a sand board to slide down the dunes.