The earth's crust contains an average of 3 parts per million of Uranium with extremes of 1 to 500,000 parts per million. A number of countries extract the ore through mining. Canada and Australia are the largest suppliers at present, accounting for roughly 50% of the world's production. Typical mining schemes consist of extracting rock (containing Uranium Oxide [U3O8]) from open pits or underground mines. In some cases, the ore is extracted by in-situ leaching. Once an amount of ore has been obtained, it proceeds to the milling process which typically consists of crushing the rock, leaching the crushed rock with acid, and recovering the Uranium Oxide from the liquid. This concentrated Uranium Oxide is referred to as yellowcake.