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
Yellowknife came from gold mining, and later government; however, because of falling gold prices and increased operating costs, the final gold mine closed in 2004, marking a turning point for Yellowknife's economy. After a downturn in the 1990s during the closure of the gold mines and the downsizing of the government workforce in 1999, Yellowknife's economy has recovered, largely because of the diamond boom; the Ekati Diamond Mine, owned and operated by BHP Billiton (sold to Dominion Diamond Corporation in 2013), opened in 1998. A second mine, Diavik Diamond Mine, began production in 2003. Production from the two operating mines in