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
Argyle diamond mine of the open pit; these reserves would be mined underground (by block caving), rather than the open pit method currently used. In late 2005, Rio Tinto Ltd concluded that the operation was economically feasible. Pre-production construction of the underground mine commenced in early to mid 2006 The Argyle diamond mine is economically feasible because its large reserves and high-grade ore offset a low average diamond value. The estimated value of Argyle diamond production is only US$7 per carat ($35/g); this compares to values of $70 per carat ($350/g) for diamonds produced at the Diavik mine and US$170 per carat ($850/g)