The Dow Jones Industrial Average fell 508 points on October 19, 1987. That was 22.6 percent, the worst single day in its history, worse than any day in 1929. The day became known as Black Monday.
The fall had been building for a week. Markets had wobbled the Friday before, and over the weekend the panic gathered. When trading opened on Monday, the sell orders came in a flood, and the new computer-driven trading made it faster than anything seen before.
Program trading was new and blamed. Machines were set to sell as prices fell, which made prices fall further, and the loop fed itself. The specialists on the floor could not keep up, and the tape ran hours behind the market. Paperwork nearly broke the system.
The crash spread. Hong Kong had already fallen, and London and Tokyo followed, and by the end of the day most of the world's markets had lost a fifth or more. Yet no depression followed. The economy kept growing, and the market recovered within two years.
The day changed how markets were run. Circuit breakers were installed to stop free-falling trading, and the computers were reined in. Black Monday was a warning that the new machinery could outrun the people running it. The warning was heard, for a while.