Usually platinum mining is underground although there is some open-pit mining. The extraction stage of platinum is labour-intensive. Miners bore holes with hand-held pneumatic drills and blast them with explosives. Afterward ore is removed and transported to the surface. Normally, mill-head grades (the measure for the PGM content for the ore) are between 4 and 7 grams per tone. Once in the surface, ore is crushed and milled in order to obtain smaller rock particles and to expose the minerals which contain the PGM. In a "froth flotation" process these particles are mixed with water and special reagents, before air is pumped through the liquid. As a result bubbles to which the PGM-containing particles adhere are created and they float to the surface. This flotation concentrate is removed as a soapy froth. The pgm content, at this stage, varies between 100 and 1000 grams per tonne. In order to obtain the maximum amount of pgm, the material, which fails to float, goes through a second milling and flotation process.