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 procuring equipment and raising funds due to World War II. By the 1950s he had developed the Williamson diamond mine into the first significant diamond mine in Tanzania. The mine was visited in the early 1950s by the US author John Gunther who describes it in his book "Inside Africa". The mine then employed 2,600 African workers, 110 Europeans and 60 Asians. The mine was noted for technical innovations in diamond mining which were developed under Williamson's watch. Williamson closely managed the mine until his death in 1958 at the age of 50. Williamson's heirs sold the mine for about