Permafrost

arctic · frozen ground · geology · climate

Super old ice. Some permafrost has been continuously frozen for around 700,000 years, making it one of the oldest things on Earth's surface.

Wildly different depths. Permafrost can be shallower than a meter (a few feet) or deeper than 1,500 meters (nearly a mile), depending on where it is.

Found everywhere frozen. Permafrost exists under vast Arctic regions and on narrow mountain summits, wherever the ground stays cold enough year-round.

The active layer trick. Just above the permafrost is an 'active layer' that freezes and thaws with the seasons, kind of like a blanket protecting the permanently frozen ground below.