Argyle diamond mine at the Ekati mine, both in Canada. However, Argyle has two to four times the concentration of diamonds (ore grade) of these mines. This makes extraction economically feasible, as mine costs are mostly related to the amount of ore processed, not the amount of diamond extracted. In 2005, Rio Tinto was given the go-ahead to a future expansion project, moving it from an open pit to an underground mine. This plan was postponed; in September 2010 Rio Tinto announced fresh plans to develop an underground mine beneath the existing pit, increasing annual production to 9 million tonnes of ore. The
Permafrost evident from temperature measurements in boreholes in North America and Europe. Typically, the below-ground temperature varies less from season to season than the air temperature, with mean annual temperatures tending to increase with depth as a result of the geothermal crustal gradient. Thus, if the mean annual air temperature is only slightly below , permafrost will form only in spots that are sheltered—usually with a northerly aspect—creating "discontinuous permafrost". Usually, permafrost will remain discontinuous in a climate where the mean annual soil surface temperature is between . In the moist-wintered areas mentioned before, there may not be even discontinuous permafrost