A phytoplankton bloom in the Baltic sea near the island of Gotland.  This is viewed in visible light -- the plankton has turned large sections of the water green, and has made water currents visible.
Crop of a Landsat 7 image, desktop size (1280x1024).  US Public Domain, free for commercial or non-commercial use.  Attributions should be made to the U.S. Geological Survey (USGS).
RemusShepherd
CC BY 2.0
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