It was a day investors won't soon forget.
On May 6, global markets were already on edge. The European debt crisis was in full swing and looked like it might reach the shores of the U.S.
Stress in the credit markets only added to the anxiety and it showed on The Street. Shortly before 2pm ET, the markets were down nearly 3 percent, but little did anyone know that what would happen in the next half-hour would shake the market to its core, rattling minds from New York to Washington.
Just after 2:30 p.m., the selloff became a full-fledged free-fall, with the Dow Jones Industrial Average falling 600 points, then 700, then 1,000 points in a matter of minutes.
Some individual stocks saw dramatic swings too. Proctor and Gamble, one of the market's most stable stocks, fell nearly 50 percent in seconds. But just as fast as stocks fell, they came back. At the end of the day, the Dow ended down 347 points, far from the lows of the session.
The whole event lasted 16 minutes and would come to be known as the Flash Crash, for the speed in which the slide and the rebound occurred.
Though the exact cause of the so-called Flash Crash is still not clear, some have focused on the activities of high-frequency traders—market participants who use high speed computers to trade millions of shares of stock in the blink of an eye.
The event also shed light on how much the structure of the markets has changed over the years.
A decade ago, most stock trading was handled on the floor of the New York Stock Exchange, a unit of NYSE Euronext. Today, it's much more fragmented. Though the NYSE and its electronic arm Arca have a 27-percent market share, and Nasdaq OMX'sNasdaq stock market23 percent, new, all-electronic platforms such as BATS, based in Kansas City, and Direct Edge, now account for roughly 10 percent each.
The rest of the marketplace is split by Electronic Communication Networks, or ECNs, so-called dark pools and dozens of smaller players.
Trading is also increasingly more automated with high-frequency trading a multi-billion dollar business accounting for up to 70 percent of the action in US stocks.
And while the industry argues the practice adds efficiency and liquidity, or billions of shares to buy or sell, to the markets, critics worry that the rise of the machines has put the entire system at risk—sowing the seeds of the next flash crash.
Virginia Senator Mark Warner is one of them.
"We may have have seen the first shot of the next financial crisis on May 6," he said. "It may be a blessing that we caught it this first time and can act now rather than waiting for the next time where even with the circuit breakers in place you could end up seeing a huge precipitous fall."