ST. LOUIS (KTVI) — More than 30 feet below the ground in St. Louis’ Forest Park sits a system of tunnels. They’re part of the River Des Peres, where over four miles of its nearly 10-mile path sit beneath the city.

The river, which was created in the 1890s, slowly became a destination for everything a river shouldn’t have. As the city was growing in the early 1900s, the smell and sight become increasingly unpleasant, especially for a major upcoming event the 1904 World’s Fair.

“Whether it was buckets at one point that people were throwing into a creek, eventually, people were piping into the creeks and streams around St. Louis and the Mississippi River. Wastewater treatment didn’t exist,” St. Louis Metropolitan Sewer District Senior Public Affairs Specialist Sean Stone said. “A lot of development around here (and) no wastewater treatment was happening. It was just going into the River Des Peres, and eventually finding its way to the Mississippi (River).”

An initial enclosure was created to try and mask the unpleasant atmosphere, but years later, a much larger, more permanent solution was installed: the tunnel system that still exists today. And it’s critical St. Louis’ wastewater system.

Nexstar’s KTVI took a tour at the intersection of multiple tunnels, nicknamed the Forest Park Junction Chain. This intersection connects multiple lines within the system, which is also where the stormwater from Forest Park meets the wastewater flowing through.

“The city has a combined sewer system, meaning wastewater and stormwater share the same pipes… that’s very common in these older major cities,” Stone said. “This heads through the River Des Peres System, drops into another tunnel later that feeds our Lemay Wastewater Treatment Plant.”

Despite the name, though, MSD officials urge residents to know that this is unlike a typical river, and strongly advise against going into the waters or entering the tunnels themselves.

“This is an active wastewater system right now. This is a mixture of what’s coming from people’s homes that includes the washing machine, their sink, the dishwasher, their shower and their toilets. You have the threat of sewer gas and H2S hydrogen sulfide,” Stone added.

When heavy rainfall comes, the water rises several feet within minutes, creating an unsafe environment. Stone said that there have been recent times the tunnels look full, especially with the consistent rain St. Louis experienced in the spring.

The design of the sewage and drainage works was deemed a National Historic Civil Engineering Landmark in 1988. Looking up inside the system, the imprints of the wood that was used as a concrete form when first originally built.

“When we build tunnels now, we’re using these giant tunnel boring machine, sometimes that are 22 feet in diameter, 500 feet long…and it’s driving and drilling a hole over miles. They didn’t do that 100 years ago. They didn’t have that. They used steam shovels and manual labor to dig this trench that they were able to put a 29 by 23 foot tunnel,” Stone said. “It’s really one of the most amazing engineering feats of a century ago.”

In terms of maintaining the structural integrity, Stone added that MSD along with specialized contractors are consistently monitoring the system. While there are no plans to expand this particular tunnel network, MSD will be expanding the system in upcoming years with 15 to 16 miles of tunnels beneath the existing ones, which would be up to 250 feet underground.

The new addition will support the existing tunnels when they reach capacity, bringing the wastewater into the Lemay Wastewater Treatment Plant. According to Stone, it would add roughly 300 million gallons of additional capacity to the entire River Des Peres System.

That project is anticipated to be completed by the late 2030s.