Sand dunes with Sangre de Cristo Mountains in the background, Great Sand Dunes National Park and Preserve, Colorado, USA

Super Sandy Sweet 16

We're in the Rockies mountain range of southern Colorado, USA, to celebrate Great Sand Dunes National Park and Preserve's 16th year as a full-fledged national park—though it was a national monument from 1932, and both the dunes themselves and the surrounding valley's history are far more ancient.

The dunes lie at the edge of the fertile, expansive San Luis Valley just east of the Rio Grande's headwaters and west of the Sangre de Cristo range shown here behind the dunes. Humans have lived around here for at least 11,000 years. But that's just a few grains in the hourglass for this sand field that formed when huge glacial lakes dried up, leaving sediments that were blown by wind against the mountains to slowly form the tallest sand dunes in North America.