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Mike Rundle, from Janesville, Wisconsin, looks at ice formations in a cave along the frozen Lake SuperiorReuters

It has been so cold for so long in the Upper Midwest of the US that the Great Lakes are almost completely covered with ice.

At Apostle Islands National Lakeshore, in Wisconsin, the rock-solid ice has allowed thousands of visitors to trek over Lake Superior to explore caves featuring dazzling ice formations. It's the first time in five years the lake surface has been firm enough to allow passage.

With no letup in the cold, the caves haven't experienced the usual thaw-and-freeze cycle, resulting in icy stalactites dangling from cave ceilings like giant chandeliers.

man and dog
Pete Miller, from Minong, Wisconsin looks through an opening with his dog MaxReuters
Sightseers standing on a frozen expanse of Lake Superior look at icicles at the mouth of a caveReuters
ice stalactites
Icy stalactites hang down from the ceiling of a caveReuters
ice wall
Sightseers photograph the ice formations on a frozen rock face along the Apostle Islands National Lakeshore of Lake SuperiorReuters
sunset frozen lake
A woman walks her dog on the thick ice covering the lake at sunsetReuters
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Sightseers are framed by icicles hanging from a cave entrance as they trek across a frozen expanse of Lake SuperiorReuters
lying under ice
Aaron Eystad from Eau Claire, Wisconsin, gazes up at ice formationsReuters
Matthias Doherty slides down a shelf of ice in a frozen caveReuters
A man photographs ice formations on a rock faceReuters
ice cave couple
Sightseers look at ice formations in a cave on the shore of Lake Superior near Cornucopia, WisconsinReuters

Sections of the lakes, which hold nearly one-fifth of the freshwater on the world's surface, freeze almost every winter. But over the past four decades, the average ice cover has progressively receded by 70%, scientists say, probably in part because of climate change. However, as this season shows, short-term weather patterns can trump multi-year trends. Winter arrived early and with a vengeance and refuses to loosen its grip.

A ship is stuck in the ice covering Lake SuperiorReuters
The propeller of a ship sticks out of the thick ice covering Lake SuperiorReuters
Sightseers trek across a frozen expanse of Lake Superior, in a scene reminiscent of a Breughel paintingReuters
travel at your own risk
A car travels the ice road on Lake Superior between Bayfield, Wisconsin and Madeline IslandReuters