Apostle Islands - Wisconsin, USA
The Apostle Islands are a group of 22 islands, located in the largest lake in the United States, Lake Superior. The islands are famous for their sea caves, most notably those on the mostly uninhabited Devils Island. Kayakers can meander freely in and out of the caves. As popular as the caves are in summer, they are also popular in the winter, when the caves ice over, and the stalactites crystallize.