Which U.S. stock index suffered the largest one-day percentage drop during the 2010 Flash Crash?

The Dow Jones Industrial Average suffered the largest one-day percentage drop during the 2010 Flash Crash.

During the 2010 Flash Crash on May 6, the Dow Jones Industrial Average fell about 1,000 points, or roughly 9%, within minutes. The episode was one of the fastest and most dramatic intraday declines in modern U.S. market history. Prices recovered much of the loss before the trading session ended.

The event began amid an already unsettled market, with concerns about the European sovereign-debt crisis. Investigations later focused on interactions among high-frequency trading, automated order systems, liquidity conditions, and a large sell order. A later U.S. investigation said a trader's algorithmic sale of E-mini S&P 500 futures contributed to the turmoil.

The crash exposed weaknesses in market structure and automated safeguards. U.S. exchanges subsequently expanded circuit breakers and adopted mechanisms such as single-stock trading pauses. The incident was not the same as a sustained bear market: its most spectacular movements happened within one trading session.