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
Hugo Dummett on a "shoestring," staked some promising ground near Lac de Gras in 1989. Dummett took a new job with BHP Minerals in 1989, and convinced his new employer to lease their property. In 1991, BHP drilling on their leased claims found micro-diamonds, sparking one of largest claimstaking rushes in mining history. Under Dummet's direction, BHP eventually developed commercial diamond reserves in five pipes; the discovery pipe turned out to be non-commercial. BHP began the permitting process for the Ekati Diamond Mine in 1995, received final approval in 1997, and opened the mine late in 1998. By 2009, annual sales from