(intriguing music) For the past several months, St.

Louis has experienced multiple large cave-ins: the one at Pappy's Smokehouse, the one at Cass and 17th, and the one on Park Avenue.

This is not a new issue, but it begs the question, why do St.

Louis's streets keep collapsing?

To understand cave-ins and why they happen, you first have to know what's unique under St.

Louis, and that is our city sewage system.

It's old like many systems in larger cities.

It's 10,000 miles of pipes, and it's a complicated system.

Jay Hoskins with the Metropolitan Sewer District says when we talk about cave-ins, you have to look at how our system is set up.

- So the older part of St.

Louis, normally we think of St.

Louis City, but also portions of St.

Louis County, is what we call a combined sewer system.

So you have much larger diameter sewers.

I think I call, tell my kids, this is the "Teenage Mutant Ninja Turtles" sewer.

So those are the big sewers where the turtles might have lived, okay?

Then that sewer pipe is large because it has to convey both the stormwater and the sanitary flow or the wastewater flow in the same pipe.

And so when it's a day when you don't have any rainfall, when the sun's shining, you have very little flow in that pipe.

But when it rains, those pipes get very, very full.

And that combined sewer system is something that's, as folks developed west, and they went out into other parts of St.

Louis County, folks said, "That's just not state-of-the-art anymore.

That's just not the way we want to go about building sewers anymore."

So they built two pipes.

They built a wastewater-only pipe and a stormwater-only pipe.

- [Olivia] According to the EPA, only around 700 communities in the U.S.

have combined sewer systems, most of them in the northeast region.

That combined system, environmental conditions, and our status as a river city impact our overall system.

We also have karst terrain, a landscape supported by eroded limestone, creating ridges, sinkholes, and other characteristic landforms.

- But those natural formations are really important to the region's geology.

When we think about all those things going on at the same time, the complex nature of our sewer pipes and the complex nature of our geology and the way that those sewer pipes work in that geology, it makes operating our sewer system very complicated.

- [Olivia] Unlike sinkholes that come from our natural environment, cave-ins come from our constructed or built environment.

So let's say something like sewage pipes faulting, redevelopment, construction changing the environment, pretty much anything that lets water go where it wasn't before can create that void underground.

Most of the time these cave-ins open up, it ends up under MSD's jurisdiction.

They're inspecting around 30,000 manholes and 300 miles of our wastewater system annually, trying to catch these cave-ins before they become a nuisance or a safety hazard.

But there's still a level of fear that can arise when people see the streets they drive, walk on, open up.

And in a city of potholes, it can be hard to tell what needs attention and what doesn't.

- If you see something, say something.

If you think that the street is collapsing near an inlet where the water would normally go, or out in front of your home where you think your sewer lateral goes through, we want to know about it.

It's not gonna help us if you put it on social media.

It doesn't help us if you put it on TikTok.

You need to call us: 314-768-6260.

That will trigger a series of motions by MSD staff to go and investigate and determine if there's something that needs to happen.