We’re a day late on this story, which is fairly well-known Chicago tale, but may not be across the country.

The Plumber’s Union Local 130 has carried out the task of dyeing the Chicago River green on St. Patrick’s Day since 1962.

Pat McCarthy, parade coordinator with the Plumber’s Union, recently told Chicago CBS affiliate that tradition happened almost by accident. Some plumbers were working with dye to detect leaks.

“One day some guys were working on that leak detection, they went to the union hall and the business manager at the time, they walked in, they were full of green dye on their shirts. He said, ‘Where the hell have you guys been? Maybe we can put that stuff in the river,’” McCarthy said.

The crews poured an orange powder into the water, making it a luminescent Kelly green. McCarthy says the formula is not shared with anyone.

“It’s a very closely held secret,” he said. “It’s very environmentally safe. It’s a very fine powder and it goes in orange and as soon as it hits water it turns a brilliant green.”

Four main on the main boat spread the “leprechaun dust” into the river, while two are in a smaller boat follow behind to churn up the river and spread the green color.

More details on the dyeing process are here.