A delicate balance of opposing winds around the Sangre de Cristo Mountains have made the Great Sand Dunes the tallest of their kind in North America. Researchers from the Center for Earth and Planetary Studies conduct research at the Dunes in the hopes to learn more about the surface formation of Mars. 

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<p>Photographer:&nbsp;Samuel F. A. Cartwright</p>

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Smithsonian's National Air and Space Museum

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