For the first time in five years, people can access this amazing wintertime phenomenon.

With nearly all of Lake Superior frozen over, tens of thousands of people are trekking to Wisconsin's Apostle Islands to explore some incredible-looking ice caves.  

As Reuters explains:

The ice caves on Superior's shoreline are carved out of sandstone by waves from the lake and derive their name from the icy freeze in winter that makes them glisten with hoar frost, icicles and ice formations.

Reachable in warm weather by boat, the caves are accessible in winter only by walking across ice when it is thick and stable enough.

The National Oceanic and Atmospheric Administration says that about 94 percent of Lake Superior is currently covered in ice. It's the first time the ice has been stable enough for pedestrians since 2009.

More than 35,000 people have made the mile-long trek. Park spokeswoman Julie Van Stappen told Reuters, "we have never had this number of people coming."

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Sightseers crouch to avoid icicles in an ice cave on frozen Lake Superior, the world's largest freshwater lake, at the Apostle Islands National Lakeshore near Cornucopia, Wisconsin February 14, 2014. (REUTERS/Eric Miller)

Matthias Doherty slides down an ice wall in a ice cave of the Apostle Islands National Lakeshore of Lake Superior, February 14, 2014. (REUTERS/Eric Miller)
Pete Miller, from Minong, Wisconsin looks through an opening with his dog Max at the sea caves of the Apostle Islands National Lakeshore of Lake Superior, February 14, 2014. (REUTERS/Eric Miller)

Mike Rundle, from Janesville, Wisconsin, looks at ice formations in a sea cave at the Apostle Islands National Lakeshore of Lake Superior, February 15, 2014. (REUTERS/Eric Miller)

Sightseers trek across a frozen expanse of Lake Superior, the world's largest freshwater lake, to the ice caves near Cornucopia, Wisconsin February 15, 2014. (REUTERS/Eric Miller)
Sightseers trek across a frozen expanse of Lake Superior, the world's largest freshwater lake, to the ice caves of the Apostle Islands National Lakeshore, February 14, 2014. (REUTERS/Eric Miller)
A man rests on frozen Lake Superior, the world's largest freshwater lake, at the ice caves of the Apostle Islands National Lakeshore, February 15, 2014. (REUTERS/Eric Miller)
Sightseers look at icicles at the mouth of a ice cave of the Apostle Islands National Lakeshore, February 14, 2014. (REUTERS/Eric Miller)
Sightseers look at a frozen rock face along the Apostle Islands National Lakeshore of Lake Superior, the world's largest freshwater lake, near Cornucopia, Wisconsin February 14, 2014. (REUTERS/Eric Miller)

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