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
Williamson diamond mine a result of this poor performance, Tanzania invited De Beers to purchase back into the mine, which the company did. In 1994 De Beers bought 75% of the Williamson mine, with Tanzania retaining the remaining shares. Since 1994, De Beers has acted to improve the performance of the Williamson diamond mine by trimming the work force, making capital investment in equipment, and bringing in expertise in technology, technique, and exploration. Although the mine is still viewed as a marginal performer due to its low ore grade, De Beers has identified avenues for development that would keep the mine operating into