Which 2010 event caused a near-record intraday plunge in U.S. stock indexes?

The 2010 event that caused a near-record intraday plunge in U.S. stock indexes was the Flash Crash.

On 6 May 2010, U.S. equity markets fell extremely rapidly before recovering much of the loss within minutes. The Dow Jones Industrial Average dropped about 1,000 points, or nearly 9%, during the session. Many individual securities experienced unusually low or high temporary prices.

Regulators concluded that a large automated sell order in E-mini S&P 500 futures interacted with existing market conditions and high-frequency trading activity. As liquidity thinned, trading systems and algorithms responded to one another, accelerating the decline. The exact market mechanics were complex, but the event demonstrated how quickly automated trading could amplify stress.

The crash led to reforms including circuit breakers and improved coordination among U.S. exchanges. It is distinct from a prolonged bear market: the defining feature was the extraordinary speed of the fall and partial recovery on the same day.