As thousands head to downtown Chicago Saturday to watch the Chicago River be dyed green for St. Patrick's Day, many may be wondering what's exactly in the mysterious dye that gives the river its bright emerald color.
The river is dyed green each year by the Chicago Plumbers Union, Chicago Journeymen Plumbers Local 130, with this year marking their 69th year taking part in the uniquely Chicago tradition.
Here's how they do it (although they don't reveal all their secrets) and a bit on the history behind one of the city's most recognizable traditions.
Why is Chicago's River dyed green?
The tradition, adhered to for decades, will coincide with the city's annual St. Patrick's Day parade.
The story goes, in 1961, a man by the name of Stephen Bailey -- the business manager of the plumbers union -- was approached by "one of his plumbers who was wearing some white coveralls," a post by Local 130 says. It was then the union says, that Bailey noticed the overalls had been stained or dyed with "a perfect shade of green," or "an Irish green to better describe it."
When wondering how the coveralls could have turned such a tint, the tale continues, Bailey and his plumbers discovered that it was from to the dye used to detect leaks in the river.
"That's when Mr. Bailey bellowed," the Union declares, "Call the mayor ... we will dye the Chicago River green!"
And there you have it.
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Who dyes the River green?
For those who have never seen the process in person, each year the Chicago Plumbers union embarks on boats that putter along the Chicago River, with a rather curious concoction that sprays dye out of plumping pipes and spout pumps.
But the mysterious mixture doesn't come out green.
"If you were watching this for the first time you would think this is a mistake or a bad joke," the union said. "You see the dye is orange, and its initial color on the surface of the river is orange, and you would think to yourself what 'heathen would do something like this.'"
However, once the dye sets in, the color "in a stroke of luck" turns, and the "true color magically appears," the union said.
WATCH: Behind the Scenes: Creating Chicago's Iconic Green River Dye
According to Local 130, other cities have attempted similar feats, but never found success.
"We believe that's where the Leprechaun comes in," Local 130 says.
"As the late Stephen Bailey has said, the road from Chicago to Ireland is marked in green," the post regales. "From the Chicago River to the Illinois River, then to the Mississippi, up the Gulf Stream and across the Atlantic you can see the beautiful green enter the Irish Sea, clearly marking the way from Chicago to Ireland."
What's in the dye?
The actual recipe has never quite been revealed, the plumbers' union won't reveal its secrets.
The plumbers union, Choose Chicago says, "still holds the river-dyeing honors today." But you won't be able to find their recipe anywhere.
There is one thing the city does say about the dye, however.
"Their environmentally friendly dye formula remains a closely kept secret," Choose Chicago says.
