This winter has been so harsh in the Midwest that the Great Lakes are almost completely covered with ice for the first time since 1994. The coast guard is working to clear paths for ships carrying essential cargo like salt, oil and coal, the Associated Press reports.
The Great Lakes hold almost one-fifth of the freshwater in the world and partially freeze over every winter. But over the last forty years, the average ice cover has decreased by 70%. The biggest exception came in 1994, when 94 percent of the lakes’ surface froze. This year, ice has covered 88 percent of their surface, according to the federal government’s Great Lakes Environmental Research Laboratory in Ann Arbor, Michigan.
Solid ice has allowed visitors to cross Lake Superior to explore the caves at Apostle Islands National Lakeshore in Wisconsin and gaze upon their otherworldly ice formations. According to AP, this is the first time since 2009 that the lake surface has been stable enough for people to reach those caves (shown in the photo above).
Though the polar vortex has kept people indoors, it may benefit the lakes. The ice limits evaporation, which allows water levels to replenish. Plus, it protects lakeside cities from large dumps of snow: cold air masses sometimes suck up moisture from the water, which turns into heavy snowfall. The icy cover prevents such heavy snowfall from lake moisture.