Scenes from the Apostle Islands National Lakeshore in Bayfield, Wis., where Lake Superior's ice is thick enough to walk to the area's sea caves for the first time in five years. Derek Montgomery for MPR News hide caption

itoggle caption Derek Montgomery for MPR News

The Cathedral Spires in the Black Hills of South Dakota are just one of innumerable formations across the planet that speak to the Earth's ancient history. K. Scott Jackson/Ohio Water Science Center/USGS hide caption

itoggle caption K. Scott Jackson/Ohio Water Science Center/USGS