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 about 1,100 staff, mostly Tanzanians. The mine is located about south of the town of Mwanza in Kishapu District of Shinyanga Region. Dr. Williamson, the discoverer of the site, first owner and namesake of the mine, named the site "Mwadui" after a local chief; "Williamson" and "Mwadui" are now virtually synonymous in the diamond mining world. The most important geological feature of the Williamson diamond mine is the kimberlite pipe on which it is located. At 146 square kilometers in area at surface level, it ranks as the largest economically exploitable diamond-bearing volcanic pipe in the world. Recent exploratory drill