On St. Patrick's Day in 1965, Chicago mayor Richard Daley did something extremely unusual to the Chicago River. This tradition carried on for many years. What was it?

This now-famous Chicago tradition began as a happy accident. In the early 1960s, city plumbers were using a fluorescent dye to trace illegal pollution sources dumping waste into the river. The business manager for the plumbers' union, Stephen Bailey, noticed that the dye stained a co-worker's coveralls a perfect, vibrant shade of Irish green. Seeing an opportunity for a unique holiday celebration, he pitched an idea to Mayor Richard J. Daley: why not turn the entire river green for St. Patrick's Day?

The city embraced the quirky suggestion, and the first dyeing took place in 1962. That initial attempt used 100 pounds of an oil-based dye, which kept the river a brilliant green for nearly a week. After environmental concerns were raised about the original substance, the parade organizers switched to a more eco-friendly alternative. Today, a crew zips across the river in motorboats, releasing about 40 pounds of a vegetable-based, orange-colored powder.

As soon as the powder hits the water, a magical chemical reaction transforms it into the iconic emerald green. The exact formula is a closely guarded secret, known only to the family of the original organizers. What started as a plumber's clever observation has become one of Chicago's most beloved and internationally recognized annual traditions, drawing hundreds of thousands of spectators to the riverbanks each March.