Mining is already a reasonably extreme activity, moving and processing large quantities of material in often unpleasant and hazardous conditions. But imagine how much more extreme it would be to mine at the bottom of the ocean or on asteroids in the depths of space. That is exactly what a few pioneering companies are planning to do.The impetus for these extreme forms of mining is the recent dramatic rises in the price of many metals, driven by dwindling supplies from conventional land-based sources and by large increases in demand outstripping available supply. This is the case for bulk metals such as copper, nickel and cobalt, precious metals such as gold and platinum, and the so-called rare earth elements such as lanthanum and neodymium that are used in many modern technologies. Since 2000, the price of copper has quadrupled and the price of platinum has tripled. The rare earth elements have collectively increased in price by a factor of 20 since 2005. This all means that potential sources of these metals that were previously dismissed as too far-fetched, such as the ocean floor and asteroids, have now become economically viable – at least theoretically.Indeed, the mineral resources potentially available on the ocean floor and in asteroids are quite staggering. It has been estimated that an area of sediment just 1km2 at the bottom of the Pacific Ocean near Hawaii contains enough rare earths to meet one-fifth of global annual demand, while a single platinum-rich 500m-wide asteroid could yield around 1.5 times the known global reserves of platinum group metals. But even though the economics of extracting metals from the ocean floor and asteroids may now be more favourable, the technical obstacles remain substantial. Despite lots of planning, no deep-sea mining operations have yet been undertaken, and the necessary technologies for asteroid mining have not even been developed. The potential environmental costs could be also be high: deep-sea mining may cause major disruption to ocean habitats, while asteroid mining could cause one to crash into the Earth.20,000 leagues under the seaOf the two, deep-sea mining is the closest to becoming a reality. The idea of extracting metals from the ocean floor actually goes back over 100 years, to the late 19th century discovery of metal-rich, potato-sized polymetallic nodules on the seabed. Since then, interest has tended to wax and wane, but is currently very high.