On October 19th, 19-87, later known as "Black Monday", the Dow Jones Industrial Average lost 508 points, more than 22% of the market's value.
The broader S-and-P 500 index also fell about 22%.
Market historians say the crash was the result of a sell-off triggered by panicked investors who anticipated a crash, following an increase in interest rates.
The all-time worst crash was the September 31st, 1931 sell-off that launched the Great Depression.
You may remember the market lost more than 777 points on September 30th, 2008.
That was the biggest ever points loss, but it's not even in the top ten percentage losses, at just seven percent.