U.S. stocks plunged sharply in the final minutes of trading Wednesday after seesawing for much of the afternoon session. The Dow Jones Industrial Average tumbled a total of nearly 240 points by the day's end. The blue-chip index leaped as much as 159 points following the opening bell after China unveiled further stimulus measures to boost flagging growth in the world’s second-largest economy.

The Dow Jones Industrial Average (INDEXDJX:.DJI) plunged 239.11 points, or 1.45 percent, to close at 16,253.57. The Standard & Poor's 500 index (INDEXSP:.INX) lost 27.34 points, or 1.39 percent, to end at 1,942.07. The Nasdaq composite (INDEXNASDAQ:.IXIC) fell 55.40 points, or 1.15 percent, to finish at 4,756.53.

Gains evaporated in afternoon trading. The timing of the volatility also coincided with a certain tech giant's highly anticipated launch event. Shares of Apple Inc. (NASDAQ:AAPL) lost nearly 2 percent Wednesday to close at $110.15 after its iPhone 6S unveiling event. Market professionals said before the event that if Apple didn’t present a “wow factor” product, shares would decline in the short term. 

“To say it’s driven solely by one stock is a little bit overblown,” said Paul Springmeyer, senior portfolio manager at the Private Client Reserve at U.S. Bank.

But the broader market's performance Wednesday came down to two issues: the uncertainty surrounding China and the U.S. Federal Reserve’s looming rate hike decision next week, Springmeyer said. Stock prices have been on a wild roller-coaster ride in recent weeks, ignited by fears of global financial contagion that might result from China’s economic slowdown. Most economists previously anticipated that the Federal Reserve would announce an interest rate increase this month for the first time in almost a decade, but some experts say its decision could come down to the wire because of the recent stock market gyrations.

However, experts still expect the stock market to trend higher by the end of the year. “September has historically been the hardest month in terms of performance returned,” Springmeyer said, adding that since 1950, September has been a negative month 45 percent of the time.

The losses came just a day after the Dow surged nearly 400 points. But as the Fed moves closer to a possible tightening of monetary policy, it will likely shift the stock market into a more volatile environment. "As we all know, during bear markets, rallies can be fierce, but usually short-lived,” Mark Newton, chief technical analyst at Greywolf Execution Partners, said in a note.

Global markets Wednesday initially were buoyed by optimistic news overseas after China's finance ministry launched more stimulus to support the country's slowing economy. Meanwhile, Japan's Nikkei index surged 7.7 percent, posting its biggest gain since October 2008, helped by the prospect of corporate tax cuts and additional stimulus in China.

All 10 sectors in the S&P 500 closed lower, led by a nearly 2 percent decline in energy stocks. Consumer staples and healthcare stocks also lost roughly 1.7 percent.

Dow component Chevron Corp. (NYSE:CVX) led the index lower, shedding 2.5 percent after U.S. oil prices fell below $45 a barrel.

Crude prices traded sharply lower Wednesday on global glut fears, with West Texas Intermediate crude, the benchmark for U.S. oil prices, down 3.6 percent to $44.27 per barrel for October delivery on the New York Mercantile Exchange. On the London ICE Futures Exchange, Brent crude, the global benchmark for oil prices, lost 3.8 percent to close at $47.63.