NASA's image of the day is a composite, taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on its Aqua satellite. It shows phytoplankton swirling in the currents of the Black Sea. Credit: NASA/Jeff Schmaltz, LANCE/EOSDIS Rapid Response

The Black Sea is one of the largest bodies of water on earth, measuring 168,500 square miles, and it turns out not to be black at all. NASA's appropriately-named Aqua satellite captured this shot last month, showing the deep blues and turquoise colors of the Sea from an orbital altitude of 438 miles. This is actually a composite image, made up of multiple photographs taken during several passes over the region.

The light-colored swirls are billions of phytoplankton – floating microscopic organisms plated with calcium carbonate.