Friday marks the 25th anniversary of Black Monday, the biggest single-day drop in the stock market's history.

On that fateful day, the Dow Industrials plunged 22.6 percent (equivalent to 508 points) as a record 604 million shares changed hands at the New York Stock Exchange — three-times the typical daily volume that year. Declining volume outpaced advancing volume by 50 to 1 that day.