A huge aircraft hangar in Boden, in northern Sweden, big enough to hold a dozen helicopters, is now packed with computers-45,000 of them, each with a whirring fan to stop it overheating. The machines (pictured) work ceaselessly, trying to solve fiendishly difficult mathematical puzzles. The solutions are, in themselves, unimportant. Yet by solving the puzzles, the computers earn their owners a reward in bitcoin, a digital "crypto-currency". The machines in Boden are in competition with hundreds of thousands more worldwide. The first to solve a puzzle earns 25 bitcoins, currently worth $6,900. Since bitcoin's invention in 2008 by a mysterious figure calling himself Satoshi Naka-moto, people have increasingly traded it for real money, albeit at a wildly varying price (see chart, next page). Although there are only $3.8 billion-worth of them in cir-culation-about twice the value of Paraguayan guaranies in use-bitcoins have three useful qualities in a currency: they are hard to earn, limited in supply and easy to verify.
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