The 30th anniversary of Black Monday

A screen above the trading floor of the New York Stock Exchange. Photo: Richard Drew / AP

Today is the 30th anniversary of Oct. 19, 1987, the "biggest single-day stock market collapse in history — a 23 percent drop" that "rendered once-trusted ideas useless and redefined the financial landscape for market professionals," Bloomberg recalls.

Why it matters: "Black Monday ... is part of financial history's fossil record, a divide between old and new markets. It was the first significant instance of computer-driven trading run amok."