Photograph by Brian Skerry
The sweeping color of sea and sky, blue is a common thread in nature, seen in the cerulean of a whale shark (pictured here), the indigo of a stormy night, and the cobalt of a peacock's feathers. Over the centuries, the hue has come to represent calm, cold, mysticism, and sadness.
Photograph by Tim Laman
A thick, swirling school of blackfin barracuda plies the blue waters off Sipadan Island, Malaysia. The fish are formidable predators, but they sometimes gather in groups for protection against sharks higher up on the food chain.
Photograph by Ralph Lee Hopkins
A group of chinstrap penguins lines the edge of an iceberg adrift in Antarctic waters. Chinstraps are among the most abundant penguins, and some colonies live on floating icebergs.