The US stock market can legally just stop trading if it falls too fast.

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Here's when that actually kicks in

Market wide circuit breakers are automatic trading halts triggered when a major index, typically the S&P 500, drops by a specific percentage in a single trading day, and they exist specifically to give panicked investors a forced pause rather than letting fear compound itself in real time. The system uses three separate thresholds, a 7 percent drop triggers a 15 minute halt, a 13 percent drop triggers a second 15 minute halt, and a 20 percent drop shuts down trading for the entire remainder of the day regardless of what time it happens.

These specific rules were introduced after the 1987 Black Monday crash and later refined following the 2010 Flash Crash, when automated high frequency trading briefly wiped out and then restored nearly a trillion dollars in market value within minutes. The underlying logic is simple, giving human traders and institutional investors a mandatory cooling off period interrupts algorithmic feedback loops and emotional panic selling long enough for calmer analysis and better information to actually reach the market before trading resumes.

Glossary | Investor.govwww.investor.gov