Permafrost is a layer of soil that has been frozen for at least two years. It is found in cold regions, such as the Arctic and Antarctica, as well as in high-altitude areas. Permafrost can be very thick, sometimes extending hundreds of feet below the surface.

It's important to note that "tundra" refers to a biome, not a specific soil layer. The tundra biome is characterized by its cold temperatures, permafrost, and lack of trees. The permafrost layer is a defining characteristic of the tundra.