Stocks plunged more than 9% on Thursday, with the S&P 500 26% below its February all-time high.

Why it matters: The potential economic impact from the coronavirus ended Wall Street's longest bull run in history while roiling stock markets around the globe.

Data: FactSet; Chart: Naema Ahmed/Axios

By the numbers: The S&P 500 and Nasdaq Composite fell 9.5%.

The Dow fell 10% — its worst single-day percentage decline since the 1987 market crash.

The VIX, a measure of stock market volatility, spiked to the highest level since 2008.

Between the lines: U.S. stock declines were so steep at market open that it tripped the New York Stock Exchange's circuit breaker for the second time this week.

Stocks rebounded sharply in the middle of the day after the Federal Reserve said it would make an eye-popping $1.5 trillion in funding available to the financial system. It said it would also widen its bond-buying program, and continue to offer billions in the overnight loans.

after the Federal Reserve said it would make an eye-popping $1.5 trillion in funding available to the financial system. It said it would also widen its bond-buying program, and continue to offer billions in the overnight loans. The stock gains soon faded.

What they're saying: The moves were made "to address highly unusual disruptions in Treasury financing markets associated with the coronavirus outbreak," the New York Fed said in a press release. The announcement followed instruction from Fed chair Jerome Powell, who was consulted by the rate-setting committee.

"This is extremely rare ... and an unusually aggressive response," Lou Crandall, chief economist at research firm Wrightson ICAP, told Axios in response to the Fed's announcement. Crandall noted the scale of the Fed's intervention was "reminiscent of the financial crisis."

The bottom line: Wall Street's tumultuous stretch continued after President Trump said Wednesday night that the U.S. would ban some European travelers from entering the country for 30 days. Plans for economic stimulus are still up in the air.