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
Letlhakane diamond mine Letlhakane diamond mine The Letlhakane diamond mine is a diamond mine located in Botswana about west of the city of Francistown. Letlhakane, meaning "little reeds", is owned by Debswana, a partnership between the De Beers company and the government of Botswana. It is the second oldest of four mines operated by the company, having begun operations in 1975. Letlhakane is of open pit construction. In 2003, the mine produced 1.06 million carats (212 kg) of diamond. The recoverable ore grade at the mine was about 0.26 carats (52 mg) per ton. The ore produced at Letlhakane used to be processed