The Chicago River has gone from a murky, polluted black to an emerald, polluted green as the city celebrates St. Patrick's Day.

Members of the Local 130 Plumbers Union have been dying the river green for more than 50 years, a tradition that has its origins in efforts to detect illegal sewage dumping into the heavily-polluted waterway.

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As reported by CBS News, plumbers used to check buildings along the river for sewage using the dye formula, which would turn green if toxic sludge was detected. After running through the pipes, the river would become green surrounding the buildings excreting the most waste.

Now, it's just something they do for fun.

According to conservationist group American Rivers, 1.2 billion gallons of partially treated human and industrial waste are dumped into the river every day.