A paper in Scientific Reports suggests there's a second stock market that's invisible to humans, running on millisecond-level machine time. High-frequency trading bots operate too quickly for humans to track, but a team of scientists examined five years of trading activity, from 2006 to 2011, and discovered over 18,000 "ultrafast extreme events," each one representing a temporary change of at least .8% in the price of a stock. Nearly all went unnoticed by the banking industry at large, as most of them had returned to the mean price within a second and a half.

The problem, the researchers say, is an issue of time perception. Human beings rarely react in less than a second's time, but a modern HFT bot can prepare trades in just 740 nanoseconds, giving time for over a million trades before the humans in the room notice that anything's happened. It's also a regulatory nightmare, since the regulators can't police something that happens too fast to see.

The most active shifts came from banking stocks, thanks largely to the extreme volatility that emerged with the banking crisis, but researchers also noticed that the microcrashes most often come just before or after larger volatility events, like aftershocks from an earthquake. But the ultrafast version of the crash is shorter and more immediate, unaffected by human logic. "Down in the sub-second regime, [HFT bots] are the only game in town," one researcher told Quartz. "It’s almost like you’re seeing them in pure form."