Isothermal Perma Frozen

Isothermal permo frost is a region where the temperature does not vary from season to season. It's far enough below the surface of the ground that it's not affected very much by what's happening on the surface, or really at all. If you keep going down further and further through the iceothermal perma frost, you will eventually reach a point where the temperature has increased to a point where soil is frost free. And so that's the end of the permafrost. Below that, if it never thaws, then it's part of the perme frost but still kind of varies in terms of temperature.