Sightseers trek across a frozen expanse of Lake Superior, the world's largest freshwater lake, to the sea caves of the Apostle Islands National Lakeshore near Cornucopia, Wisconsin, on February 15. According to the National Oceanic and Atmospheric Administration (NOAA) Great Lakes Environmental Research Laboratory, about 94 per cent of Lake Superior is covered with ice, enabling thousands of people to visit the islands on foot for the first time since 2009. The Great Lakes, which contain one fifth of the world's surface fresh water, are 88 pr cent covered with ice. Reuters/Eric Miller