On March 17, millions of Irish Americans celebrate St. Patrick’s Day by wearing green. The city of Chicago, Illinois, takes the tradition a step further. Since 1962, it has dyed the Chicago River green! 

Before the city’s annual St. Patrick’s Day Parade, about 40 pounds of dye are dumped into the river. While the dye is safe for the river, most people don’t know what’s in it.  

“It’s a secret,” Michele Scaccia-Coyne says. She works with the group that turns the water green. “The color can last several days, but depends on the wind and the temperature of the water.” 

The river isn’t the only landmark to go green this holiday. In New York City, the Empire State Building glows with green lights. And fountains at the White House in Washington, D.C., are also dyed green.