The Chicago River flows green once every year. Credit: Wallula Junction, Flickr

The global urge to "go green" may have reached a fever pitch in the past few years, but it was the push to protect our waterways that gave Chicago one of its best-known traditions.

The Chicago Journeyman Plumbers Local 130 call it the eighth wonder of the world: Dyeing the Chicago River green on the morning of every St. Patrick's Day Parade.

"Plumbers, they can do anything they want to do," jokes Kevin Sherlock, parade coordinator for the plumbers union, which has been putting on one of the world's biggest St. Pat's celebrations for more than fifty years. The Chicago River once flowed into Lake Michigan, the lake that provided drinking water for all of Chicago, Sherlock tells Holidash. Thanks to the plumbers, the river was re-routed more than a century ago to prevent pollution of the water source. These days, the lake flows into the river, keeping the polluted water away from the potable water.

What does that have to do with dyeing the river green for St. Patrick's Day? Well, it was the plumbers who set out to determine exactly how the river was being polluted in the 1960s, a job that heralded the introduction of the first batch of green dye to the 156-mile waterway.

"The plumbers were testing plumbing systems to see if anything was being illegally dumped into the river," Sherlock explains. "They were putting a green dye into the systems of each building." If the dye ended up in the river, they'd know whose system needed an overhaul.

The idea of adding massive amounts of green dye to the Chicago River on St. Patrick's Day, however, came from a plumber working on the project, who mentioned it to his co-workers when he stopped off at the offices of Local 130 to pay his dues. What better tribute to the Emerald Isle, after all, than a bit of Irish green in the river? The first time the plumbers tried it, the people of Chicago observed the bright green water flowing by their buildings for three days.

Today it's more like three hours before the green stuff dissipates naturally. The process begins at 10:45 a.m. on Columbus Drive on the morning of the Chicago parade. Men in giant white suits and masks throw an orange powder -- one of three colors, along with white and green, on the Irish flag -- that Sherlock likens to Tang into the river. Boats churn the water to turn it a perfect emerald green, and the current carries the color along the parade route just in time for step off at noon.

The river-dyeing festivities alone attract anywhere from 30,000 to 50,000 people -- two-thirds of whom traditionally stick around and join the even bigger crowd at the parade. "You don't need to be Irish to appreciate it!" explains former resident Karen Reilly. "Every time I saw the green water it made me smile."

So what's inside those buckets of "Tang"? Some say it's fluorescein, the same dye used by crime scene investigators to find latent blood stains, but Sherlock won't say.

"It's like the McDonald's Big Mac sauce, it's a secret," he tells Holidash with a laugh. "We can't tell you -- this is very unique to Chicago."

Even when the city of Dublin, Ireland came calling with a request for help in dyeing the storied River Liffey green, the folks at the plumber's union would only agree to assist if it was their crew who did the job -- the same group who has been making the magic happen in Illinois since the 1960s.

So what can Sherlock say about the powder? Don't worry about those white suits and masks -- it's 100 percent safe. After all, a project born of an effort to clean up the rivers would hardly be appropriate if it were toxic.

"As plumbers, we're here to protect the water," Sherlock says. "What's going in there does not harm the fish, does not harm the plant life." It's even been tested by the Illinois Environmental Protection Agency to ensure there's only fun -- and nothing funky -- about this St. Pat's tradition.