THE "biggest bubble in human history comes down crashing," tweeted Nouriel Roubini, an economist, gleefully. After an exhilarating ride skywards in 2017, investors in crypto-currencies have been rudely reminded that prices can plunge earthwards, too. In mid-December the price of bitcoin was just shy of $20,000; by February 6th, it had fallen to $6,000, before recovering a little (see chart).
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