Lake Superior Ice Caves (Wisconsin, USA)
The ice caves that form along Wisconsin's Apostle Islands National Lakeshore are only accessible by foot when Lake Superior is sufficiently frozen, and 2014 marks the first time in five years that conditions have been right. The caves have been a huge draw, with tens of thousands of visitors having already made the trek this year. After a short hike across the lake's frozen surface, visitors are able to enter caves ringed by massive icy overhangs and filled with frozen stalactites. Lake Superior's southern shore also happens to be one of the best places in the continental U.S. to view the aurora borealis, so those that brave the tundra at night may even be treated to a light show.