The Black Monday stock market crash of October 19th, 1987 was the largest one-day percentage decline in the Dow Jones Industrial Average. The crash was a genuinely perplexing event. To informed observers it seemed to have little basis in economic fundamentals. There were various “hand-waving” theories, including that the introduction of automated trading on the Dow had injected instability into the market. However, at the time, Black Monday appeared to come out of nowhere.