Permafrost is any ground, either of rock or soil, which is perennially frozen. Continuous permafrost refers to areas which have a continuous layer of permafrost. Discontinuous permafrost occurs in patches. It is believed that continuous permafrost covers approximately 4% of the earth's surface and can be as deep as 3,281 ft (1,000 m), though normally it is much less. Permafrost tends to occur when the mean annual air temperature is less than the freezing point of water. Permafrost regions are characterized by a seasonal thawing and freezing of a surface layer known as the active layer which is typically 3–10 ft (1–3 m) thick.