The earliest form of mining involved following seams of metal in tunnels driven into the rock. This method is still used today, usually in deep mines where other techniques would be impracticable. Tunnels are dug with explosives and with automatic machines. Some of these mines go thousands of metres into the rock, becoming very hot and dangerous.
Placer mining uses huge floating dredgers to extract metals such as tin and copper from submerged mud. It extracts large amounts of metal inexpensively, but causes enormous environmental damage. Strip mining is used to obtain coal and minerals that lie close to the surface. Open pit mining involves blasting into the rock to produce a huge quarry from which material is removed, layer by layer.
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