Permafrost by permafrost. Of this area slightly more than half is underlain by continuous permafrost, around 20 percent by discontinuous permafrost, and a little less than 30 percent by sporadic permafrost. Most of this area is found in Siberia, northern Canada, Alaska and Greenland. Beneath the active layer annual temperature swings of permafrost become smaller with depth. The deepest depth of permafrost occurs where geothermal heat maintains a temperature above freezing. Above that bottom limit there may be permafrost, whose temperature doesn't change annually—"isothermal permafrost". Permafrost typically forms in any climate where the mean annual air temperature is less than the
Jwaneng diamond mine of waste rock. The mine is located on three kimberlite pipes that converge near the surface, covering 520,000 square metres at ground level. Currently, the mine produces approximately 11 million carats (2,200 kg) of diamonds. The recoverable ore grade at the mine is about 1.25 carats per ton (250 mg/t). , known reserves will allow production at current levels to continue for 27 years. The high rate of diamond extraction, combined with high quality diamonds fetching excellent per weight prices, make the Jwaneng diamond mine the richest diamond mine in the world by value of recovered diamonds. Jwaneng employs over