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
Letseng diamond mine diamond ever found. It is a type II D colour, with potential to yield a cut stone. Chief executive Clifford Elphick said: "Preliminary examination of this remarkable diamond indicates it will yield a record-breaking polished stone of the very best colour and clarity, and has no inclusions visible in its rough form." The discovery of two large rough white diamonds was announced in November 2010. They weighed 196ct and 184ct. In January 2018 Gem Diamonds announced the largest diamond yet discovered at Letseng,a Type IIa D color stone measured at 910 carats. Letseng diamond mine The Letseng diamond mine, found