This image is a rare satellite view of a cloudless summer day over the entire Great Lakes region. The Great Lakes comprise the largest collective body of fresh water on the planet, containing roughly 18 percent of Earth's supply. Only the polar ice caps contain more fresh water. The region around the Great Lakes basin is home to more than 10 percent of the population of the United States and 25 percent of the population of Canada.
Open water appears blue or nearly black. The pale blue and green swirls near the coasts are likely caused by algae or phytoplankton blooms, or by calcium carbonate (chalk) from the lake floor. Photo courtesy of NASA.