In this February 2, 2014 photo, people visit the caves at Apostle Islands National Lakeshore in northern Wisconsin, transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. The caves are usually accessible only by water, but Lake Superior's rock-solid ice cover is letting people walk to them for the first time since 2009. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

Visitors take pictures of each other in a cave at Apostle Islands National Lakeshore in northern Wisconsin. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

Spiky icicles in the cave at Apostle Islands National Lakeshore in northern Wisconsin. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

In this February 2, 2014 photo, icicles hang from the ceiling of a cave at Apostle Islands National Lakeshore in northern Wisconsin, which has been transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

In this February 2, 2014 photo people visit the caves at Apostle Islands National Lakeshore in northern Wisconsin, transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

Icicles formed in caves are also known as ice stalactites. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

In this February 2, 2014 photo people visit the caves at Apostle Islands National Lakeshore in northern Wisconsin, transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

In this February 2, 2014 photo, icicles hang from the ceiling of a cave at Apostle Islands National Lakeshore in northern Wisconsin, which has been transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

In this February 2, 2014 photo people visit the caves at Apostle Islands National Lakeshore in northern Wisconsin, transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

In this February 2, 2014 photo people visit the caves at Apostle Islands National Lakeshore in northern Wisconsin, transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

In this February 2, 2014 photo, people take pictures inside a cave at Apostle Islands National Lakeshore in northern Wisconsin, which has been transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)