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 Letseng diamond mine The Letseng diamond mine, found in the landlocked Southern African kingdom of Lesotho, is owned by Gem Diamonds, Ltd. and the government of Lesotho, and at an elevation of 3,100 m (10,000 ft) it is the world's highest diamond mine. It is characterised by extremely low grade ore (less than /hundred tons) and is known for producing huge diamonds, having the highest percentage of large diamonds (greater than ), giving it the highest dollar value per carat of any diamond mine. The world average is roughly US$81 per carat, while Letseng averaged over US$1,894 per carat for