Chicago has a tradition of dying the Chicago River green in the spirit of St. Patrick’s Day. It began in 1962, a practice that has lasted exactly 60 years.

But where did this tradition originate from?

According to the Illinois Office of Tourism, the tradition began the year before in 1961, when “the business manager for the Chicago Journeymen Plumbers Local Union noticed that a plumber’s white overalls had bright green stains all over it, seemingly matching the emerald green often associated with Ireland.”

The green stains were from a dye that the plumbers used to find leaks in pipes, this inspired them with the idea to incorporate it into something larger the following year.

When the next St. Patrick’s Day came around, the Plumbers Local Union dumped 100 pounds of the emerald green dye in the river. It turned the whole river green and lasted for a week.

Now, an eco-friendlier dye is used that is made from vegetables and is less harmful to anything living in the water. It was introduced by environmentalists in 1966. Although the formula has been kept a secret for years, it is known that it is an orange powder that eventually turns green when it hits the water, according to Mental Floss.

Local residents claim this is the leprechauns' "magic" that makes the color change happen.

Today’s dying process uses about 40 pounds of dye it lasts for a few hours. It is usually dyed the weekend before St. Patrick's Day. The river is dyed by a small team of people in two boats that includes some family members of the original dyers abroad. The team uses one boat to dump the dye and the other to mix it.