Last year, I drove from St. Louis to meet up with a few friends in Chicago when I noticed something strange upon entering the Windy City.

The Chicago River was neon green, and there were hundreds of thousands of people gathered on the banks.

The river has never had good reputation for being sanitary, but the hue I saw that day caught me by surprise. Then it dawned on me: It's St. Patrick's Day!

Luckily, I arrived on the perfect day! I really appreciate the city's creativity on this festive occasion.

Every March 17, people in Ireland gather to worship the death of Saint Patrick, who was a Christian missionary and bishop in Ireland.

In the U.S., we celebrate by wearing green, which symbolizes spring, and going out for a drink.

In Chicago, the river is turned green, leaving thousands of people wondering when this tradition started, and how they actually dye the river...

History

This tradition got its start way back in the early 60s, but was inspired by something that happened years earlier.

The former Mayor of Chicago, Richard J. Daley, hated how the Chicago River looked. It was filled with sewage, and it was his goal to figure out why it looked that way.

A green dye was poured into the river, which was able to help them find out where the sewage was coming from.

So by the early 60s, someone got the idea to make the the whole river turn green for St. Patrick's Day.

They poured 100 pounds of Ecto Cooler, a toxic-green liquid, into the river. The problem was that the river stayed emerald green for a week, and over time they learned the perfect amount to pour in.

Then came their next problem: Environmental activists were opposed to the chemicals that were poured in, which were starting to pollute the river.

Today

Now the water is dyed using a powdered, vegetable-based dye that is actually orange. When the power is mixed with water, it turns green. (But no one wants you to know that!)

So the next time you're in Chicago on St. Patrick's Day, go out on the bank at 9 a.m. of the parade to watch the Plumbers Union pour the dye in the river.

It's the process of the river changing color that's amazing!

It takes less than an hour for the river to turn green, and the strange sight only lasts for five hours.

If you can't make it this year, watch the color-changing process here!

Have you been in Chicago on St. Patrick's Day?

[H/T: Mental Floss]