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."