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The arctic weather in the Midwest has one perk: People can now visit the caves at Apostle Islands National Lakeshore in northern Wisconsin.

For the first time since 2009, Lake Superior has frozen over enough for visitors to walk on and see the sea caves, which are now called ice caves. When the lake starts to freeze, the wave action from the lake creates stunning ice formations inside the two-mile-long caves.

"It's been so cold you get some unique formations," said Julie Van Stappen, chief of planning and resource management of Apostle Islands, part of the National Park Service, in an interview with USA TODAY Network.

There have been 60,000 visitors to the lakeshore since the lake was deemed safe to walk on Jan. 15, Van Steppen said. That amounts to one-third of the lakeshore's typical number of visitors for a year.

It's unclear how much longer the lake's ice will hold up for visitors to walk out on, as ice must be at least 8 inches thick for visitors to do so. Currently, it's about 16-18 inches thick, Van Steppen said.

Apostle Islands National Lakeshore offered these tips for visiting the ice caves:

Be prepared to walk: From the parking lot, visitors still have to walk a mile to get to the beginning of the caves. Van Steppen recommended wearing cleats or at least a sole that grips ice.

Dress warmly: Single-digit highs have been the norm, Van Steppen said. It's especially important to keep your face covered because of the strong winds, she added.

Watch where you stand: Some icicles get as big as a dump truck, Van Steppen said.

Call ahead of time: As the weather warms, the ice could loosen. Call the "ice line" at 715-779-3397 ext. 3 to get an update on conditions.

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