Sometimes we go to war with our neighbors, and sometimes those neighbors are rats.

Okay, so we're outside in New York City looking at what we call active rodent signs, or A-R-S.

That is Bobby Corrigan. He is an urban rodentologist, a former rodent researcher who now works for the city of New York.

Everyone thinks there's a rat world below our feet. And to some degree, that's true. But rats have a very specific subterranean environment they need.

It is a cold and windy afternoon in Lower Manhattan, one of the oldest parts of the city.

Most of the humans have scurried back to their offices from lunch.

At the intersection of Murray and Church Streets, Corrigan points to a sidewalk curb that has collapsed in on itself.

And that's because the rats nearby got below the sidewalk, tunneled into this area,

dug out the soil so they could have a burrow in this area.

And now there's nothing supporting these heavy concrete pieces.

It's expensive to put in a new curb.

And where did these burrowing rats come from? Just five feet away,

we have the proverbial catch basing that the stormwater drains down, and sometimes you'll see

rats come right out of these sewers. Their home is in the sewer in the middle of the street.

So you've got rats in the sewers, rats burrowing under the sidewalks.

What else can we see?

I want to show you something much more interesting.

You'll notice along this building perimeter, if you let your eyes just continue along,

you will see the gray concrete that's light.

But next to the building, you'll see this dark charcoal stain that's linear, right?

The stain goes around, hugs the building, that is from rats.

That's what's called the sebum stain.

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