For the first time since 2009, the coastline of Lake Superior has frozen hard enough that people can venture out onto the ice and into the sea caves that line the shore near Wisconsin's Apostle Islands. Like the Lake, itself, the sea caves are frozen and covered with sparkling icicles — from dainty needles to thick, massive stalactites.
These are different caves from the ones I went through in a dinghy in the summer a couple of years ago. Those caves were at Devil's Island, about 6 miles from the mainland. The caves you can see in this, and several other videos taken by YouTuber Shannon Kowalski, are right up along the mainland shore, at the base of some steep sandstone cliffs. The cliffs themselves are the remains of a sandy river basin and chains of shallow ponds that dotted the landscape here a billion (yes, with a "b") years ago. The caves are much more recent, forming as waves from Lake Superior slowly erode holes in the sandstone.
You can read more about visiting the caves at the National Park Service site:
Visiting the caves in winter requires at least a 2 mile hike (round trip) on the ice of Lake Superior. Cold temperatures can form thick ice, but wind and waves can break up that ice and make it very unstable. The conditions at the caves can change in less time than it takes to walk there.
You should also check out more of Shannon Kowalski's sea cave videos:
Video 1 — featuring some amazing, feathery ice formations
Video 2 gives you a good idea of how big the interiors of some of these caves can be
Video 3 shows you the shoreline and the cliffs
Video 5 is the one I have embedded at the top of this post, with more great shots of the shoreline
Video 6 looks like a great place to encounter a Wampa