A view of the horizon on the Great Barrier Reef, Australia. The blue water in the bottom part of the frame is deeper water with a mostly sandy bottom. Beyond that, the darker area is a mix of shallow reef and sand just below the surface of the water. The shallow reef provides a natural barrier against waves and swells. This particular shot was taken about 150 miles northeast of Gladstone on Swains Reef, about 115 miles offshore.
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