Photo @materas // Ice cave beneath the Mendenhall Glacier, Tongass National Forest, Alaska. Ice caves form as meltwater runs underneath the glacier, melting the glacier from the inside. The 13 mile long Mendenhall Glacier is an outlet glacier from the Juneau Ice field, and has retreated 1.75 miles in the past 90 years and continues to retreat. Follow me @materas for more images like this from Alaska and around the world.