This is 99% invisible. I'm Roman Mars. The Chicago River used to be completely filthy. I mean, it's not great now, but it used to be so much worse. The people of Chicago were doing disgusting things to it.

Essentially, using the river as a sewer and all of Chicago's waste, human waste, and also as a stock yard screw up in the city, all of the animal waste was jumped into the river and it was said that the river was so thick with filth that a chicken could walk across it without getting her feet wet.

That's Elizabeth Colbert, author of the new book Under a White Sky.

What needs to be also understood is that the Chicago River, in its original incarnation, it flowed eastward into Lake Michigan, which was and still is Chicago's sole source of drinking water.

So to fix this problem, the city of Chicago carried out a massive project. They reversed the river and sent the sewage water into the Mississippi.

When this enormous construction project was completed, which was one of the most enormous, concerted projects of its era, there was a facetious headline in the New York Times that said something like water in the Chicago River resembles liquid again.

By the way, if you want to know more about how it's even possible to reverse the river, we did a whole story about it, episode 86. It's a good one.

Anyway, the river reversal was a big success. Chicagoans had a reliable supply of clean drinking water. But like many large-scale human interventions, there were unintended consequences.

The reversal meant two unconnected drainage systems, the gray legs and the Mississippi, were suddenly linked up and that invasive species could move from one to the other.

Both the gray legs and the Mississippi system became highly invaded water systems, especially the gray legs. The gray legs are like a 180-known invasive species established in the gray legs.

And the species of interest right now, sort of as it was put to me by one engineer with the Army Corps of Engineers, is Asian carp.

If Asian carp reached the gray legs, they get pretty much ruined everything. They would eat all of the endangered mollusks and even threaten the safety of human beings.

One of the species has the very annoying habit from a human perspective of flinging itself out of the water when it's disturbed.

And what disturbs it often is a boat, a motor, a sound of a motor. And so you get, you know, man-v fish, you get a lot of injuries.

You know, I met people whose eye sockets had been broken by Asian carp.

Faced with this epidemic of fish jumping out of the water and smacking people in the face, the Army Corps of Engineers were told, well, you have to fix this. And they came up with a series of plans to stop the carp from migrating up the Chicago River, like zapping the river with UV radiation or putting in a big filtration system or dumping nitrogen in the water to basically poison the fish.

But what they eventually arrived at was the idea of setting up an electric barrier in the water.

This underwater sort of ushaped structure that has these nodes in it that just causes a lot of voltage through there.

So just a recap. We dumped sewage into a river, then reversed its flow by connecting it to an entirely different river, and finally we electrocuted the river.

This is something humans do a lot. We metal with nature. And years later, we discover that that creates a whole bunch of unintended consequences. And then we have to metal with nature all over again.

These kinds of interventions are the subject of Elizabeth Cobert's new book. Today we're going to talk with Cobert about all the extreme links humans will go to try to undo our mistakes.

There are so many examples of situations where people intervene to undo the consequences of previous interventions. But one that really caught me in your book is the introduction of cane toads in Australia.

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