This 85,000-acre national park in southern Colorado is home to the tallest sand dunes in the U.S. — reaching 750 feet. There are no designated trails for visitors to use on the dunes; you may explore any part. The only advisory given is to be careful of very hot sand in the summer (and the dunes get snow in the winter). In addition to dunes, there are alpine lakes and tundra, high mountain peaks, forests, grasslands, wetlands and many diverse species of plants and animals. The dunes also create a rare hydrologic phenomenon: creeks that appear to flow on the surface of the sand.