"Black Monday, October 19, 1987, day on which stock market prices in the United States fell precipitously, accelerating steep losses in stock markets around the world. By the end of the day the Dow-Jones Industrial Average had fallen more than 500 points. Economic observers blamed the crash on the lack of international trade leadership by the federal government, weaknesses in the U.S. economy, and computerized trading on Wall Street. The crisis eased during the week following Black Monday."

For perspective, the NYSE was floating around at 2000 points. I remember it breaking 3000 some time during the early ninties. Today the market closed at 10963.80. So in order for something similar to occur, we would be talking a drop of around 2500-3000 points.