October 19th marks the 30th anniversary of what's called "Black Monday" when the Dow Jones had its biggest one-day percentage drop in its history; it was one of the worst days in trading in Wall Street ever. To this day, there’s still no agreement as to what exactly sparked the crash, with historians and government officials pointing to a variety of contributing factors. But a major suspect was an investment strategy known as portfolio insurance.

"We came so close to a system cracking meltdown, fuse blowing crisis. We need to know that," said Diana Henriques, a financial journalist and author of the new book A First-Class Catastrophe: The Road to Black Monday, the Worst Day in Wall Street History. 

This is the first of several stories in WNYC's Crash Course, a series about the stock market crash of October 19, 1987, and what it can tell us about today's financial markets. If you've ever regretted a decision about investing in the stock market, how did you make that choice? Tell us your story.