For a truly unique caving experience, go no further than the shorelines of Apostle Islands in Wisconsin's Lake Superior.
In the summer, kayaking along the shore enables visitors to observe each exquisite cavern. When the lake freezes over in the winter, the caves follow suit. Visitors trade in their kayaks for rugged snow boots, and trek off on foot to see frozen waterfalls and enormous icicles.
Apostle Islands National Lakeshore isn't known for more than its striking caves; the lakeshore's ecosystem is one of kind. Numerous species of plants and wildlife call these islands home, including a large concentration of black bears on Stockton Island and many nesting birds.
To see what the caves look like in the summer, check out the following video: