Wired reports: The afternoon of May 6, 2010 was among the strangest in economic history. Starting at 2:42 p.m. EDT, the Dow Jones stock index fell 600 points in just 6 minutes. Its nadir represented the deepest single-day decline in that market’s 114-year history. By 3:07 p.m., the index had rebounded. The “flash crash,” as it came to be known, was big, unexpected and scary — and a new study says flash events actually happen routinely, at speeds so fast they don’t register on regular market records, with potentially troubling consequences for market stability.

The analysis involved five years of stock market trading data gathered between 2006 and 2011 and sorted in fine-grained, millisecond-by-millisecond detail. Below the 950-millisecond level, where computerized trading occurs so quickly that human traders can’t even react, no fewer than 18,520 crashes and spikes occurred. The study’s authors call those events “financial black swans,” though they’re so common that the black swan label probably doesn’t fit anymore.

Moreover, those events fell into patterns that didn’t fit market patterns seen at other time scales. It’s as if computerized trading has created a new world, one where the usual rules don’t apply, populated by algorithms and only dimly understood by the people who made them. The extent to which that world influences our own — perhaps making events like the 2010 flash crash more likely, or causing markets to be generally more volatile — is an open question.

“There’s this whole world below 650 milliseconds. It’s like landing on another planet,” said Neil Johnson, a complex systems specialist at the University of Miami and co-author of the study, released Feb. 7 on arXiv [PDF]. “It’s an enormous part of the market which is out of human reach. We have a glimpse of the kind of ecology that’s going on down there.”