On Saturday (March 16), Chicago celebrated its world-famous St. Patrick's Day event, with the 'Windy City' river gleaming in a vibrant shade of emerald.
The festive occasion drew thousands of spectators from around the world, who marveled as the Chicago River transformed into a luminous green canvas.
In a tradition that began in 1962 and is now in its 69th year, the Chicago Plumbers Union injected 50 pounds of green dye into the river, covering a quarter-mile stretch with a vibrant, algae-like color.
The union assured that the eco-friendly dye would not harm wildlife and would retain its vibrant color for about five hours.
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.