spotted a fascinating story at The Atlantic about "mysterious and possibly nefarious trading algorithms
[that] are operating every minute of every day in" the stock market:
"Unknown entities for unknown reasons are sending thousands of orders a second through the electronic stock exchanges with no intent to actually trade. Often, the buy or sell prices that they are offering are so far from the market price that there's no way they'd ever be part of a trade. The bots sketch out odd patterns with their orders, leaving patterns in the data that are largely invisible to market participants."
Spotting the behavior of these bots was possible by looking at much finer time slices than casual traders ever see — cool detective work, but as the story points out, discovering it is just the beginning: "[W]e're witnessing a market phenomenon that is not easily explained. And it's really bizarre."