The hard rocklike material on which the living algae Clathromorphum compactum sits are layers of calcified crusts the algae deposit year after year. These “red rocks” grow in shallow water (49 to 55 feet deep) and are widely distributed in the Arctic and sub-Arctic Oceans. Scientists from the University of Toronto, the Smithsonian and other research organizations have recently discovered how to use these crusty layers to track changes in Arctic sea ice over centuries. Each year C. compactum can lay down a crust one-tenth to two-tenths of a millimeter thick. “They grow very slowly so if you get a specimen that is 8 or 9 inches thick you are talking about something that is well over 1,000 years old,” explains Walter Adey, algae expert at the Smithsonian’s National Museum of Natural History. When viewed under a scanning electron microscope the layers offer an archive of the levels of sea ice in the Subarctic dating back centuries. This newly tapped ‘archive’ shows a dramatic decrease in ice cover during the last 150 years.