For the 55th year in a row, Chicago River went green Saturday for St. Patrick's Day.
But did you know this colorful tradition started by accident?
The official March for Science Chicago Twitter account explains.
Chicago plumbers poured 100 lbs of an oil-based fluorescein to find sources of illegal dumping.
Fifty five years later and the river dyeing tradition is still going strong.
Some people snapped photos of the green water.
Green water in Chicago is cool, but WHAT IF we used the $ spent on the dye to help out Flint, MI who hasn't had clean water in 1,051 days?— Kevin Long (@_kevinplong) March 11, 2017
Others used it as an opportunity to remind people about the Flint water crisis.
You can read more on this Chi town tradition in these archived newspaper clippings here.