This Month in Photo of the Day: Nature and Weather Photos

Water dark with tannin inspired the name Rio Negro, or "black river," which swirls across virgin sand in Lençóis Maranhenses National Park. In the park's ponds, thriving communities of algae can turn the water blue or green.

See more pictures from the July 2010 feature story "A Sea of Dunes".

See more pictures of freshwater »
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