1. The Great Lakes, which hold nearly one-fifth of the surface fresh water in the world, are almost completely frozen over after an unusually cold winter.
The Mackinac Bridge, which connects Upper and Lower Michigan, spans an ice cover that stretches into the horizon on Feb. 11, 2014.
2. As of Thursday, nearly 90% of the lakes are under a cover of ice, according to the Great Lakes Environmental Research Laboratory.
The pier and lighthouse at Lake Michigan’s Little Traverse Bay at Petoskey, Mich., is surrounded by thick ice on Feb. 11, 2014.
3. The last time they came this close to being completely frozen over was in 1994, when 94% of the lakes’ surface was ice.
The ice covered St. Clair River is seen at the Canadian shoreline on Feb. 6, 2014.
4. If the rate at which ice has been growing continues, an all-time record could be set later this month.
An aerial photo of Lake Huron looking south towards Port Huron, Mich. shows some open water on Feb. 6, 2014.
5. “In the last one to two weeks, we’ve seen rapid accumulations on Lakes Superior, Huron and Michigan,” Michigan State University associate professor and state climatologist Jeff Andresen said.
U.S. Coast Guard icebreaker Mackinaw maintains a shipping lane on the St. Marys River linking Lakes Superior and Huron.
6. The freeze is a major reversal from last year, when the five lakes only reached 38% ice cover.
The U.S. Coast Guard Cutter Biscayne Bay, a 140-foot ice-breaking tug, sails Lake Michigan near Chicago on Feb. 12, 2014.
7. The lakes’ water supply has been far below average in recent years, and the ice could prove beneficial by slowing evaporation and shutting off lake-effect snow.
The St. Joseph Lighthouse rises behind mounds of ice formed along the shores of Lake Michigan on Jan. 21, 2014.
8. When the lakes freeze, the U.S. Coast Guard’s icebreakers cut paths for ships that carry coal, salt and heating oil.
A U.S. Coast Guard icebreaker passes Chicago’s Navy Pier, left, on it’s way to Indiana in Lake Michigan on Feb. 12, 2014.
9. This winter, they’ve logged four times more hours than the recent year average, a U.S. Coast Guard spokesman said.
A yardstick measures the thickness of ice in the Straits of Mackinac on Feb. 5, 2014.
10. In Wisconsin, frozen Lake Superior has allowed thousands of people to walk miles on the ice to explore caves at Apostle Islands National Lakeshore.
People visit the caves at Apostle Islands National Lakeshore in northern Wisconsin on Feb. 2, 2014.
11. The extraordinary caves are normally only accessible by water, but ice cover has allowed visitors to walk to them for the first time since 2009.
Ice dangles from caves in the Apostle Islands on Feb. 2, 2014.