If you’ve noticed that the number of extreme events spikes around the time of the financial crisis, and the stocks most likely to experience them are bank stocks, you’ll see why the researchers are so interested in this hidden market: This pattern suggests the coupling between extreme market behaviors and global instability—”how machine and human worlds can become entwined across timescales from milliseconds to months”—and is also are seen more often before and after the kinds of “flash crashes” that people actually notice.  Regulators, though, aren’t keeping track of these events. That’s a problem, not just because of any potential forewarning, but also because trading at that speed creates volatility that makes markets less efficient.  How High Speed Trading Impacts the Market