The magic of mining

Minting the digital currency has become a big, ruthlessly competitive business.
 
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 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 "cryptocurrency".
 
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