On September 24TH the price of a single bitcoin, the best-known cryptocur-rency, fell by $1,000 in 30 minutes. No one knows why, and few people cared. There have been similar drops nearly every month since May. Yet for one obscure corner of the market, it mattered. Exchanges that sell "long" bitcoin derivatives contracts, with which traders bet that prices will rise without buying any coin, soon asked punters for more collateral. That triggered a stampede. By the end of the day $643m-worth of bitcoin contracts had been liquidated on BitMEX, a platform on which such contracts trade. Bets on other crypto-currencies also became toxic.
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