MINNEAPOLIS (WCCO) — Before the Twin Cities had skyway systems or mega malls, we had tunnels. There are hundreds of them, with many of them man-made in the 1800s.

READ MORE: Como Park H.S. Student About To Take Flight As J-ROTC Cadet

WCCO’s Jamie Yuccas had a rare chance to go inside the historic Pillsbury A Mill in Minneapolis. While it was dark and somewhat dangerous, the tunnel has a fascinating past and uncertain future.

Greg Brick explores tunnels and has delved deep into the subject for 20 years.

“The kind we’re going to be visiting today is a hydro power tunnel, and that was, interestingly enough, one of the very earliest kinds of tunnels. They predate the civil war in the St. Anthony Falls area,” said Brick.

With each step, there are new discoveries in the Flour Mill, which was built in 1881.

“A lot of times when (the tunnels) went out of use, as this one did, they were walled off, so people completely forgot about them” said Brick.

Once inside the belly of the building, Kit Richardson, an architect, is our guide.

“So, we’re now standing outside the building under Main Street and the tunnel is 660 feet long,” said Richardson, who said he feels connected to the first flour mill to ever be built by an architect.

READ MORE: Behind-The-Scenes Of Wildlife Science Center's Mission To Learn All About Wolves

The floor of the tunnel resembles a river bed. There are signs of life throughout the tunnel, including this underground garden we found where roots are actually growing through the ceiling.

“When the tunnel was closed off in ‘54-‘55, when they stopped using it for water power … there was a very small gate valve,” said Richardson.

That’s how fresh water creatures have ended up here, including freshwater clams, tiny shrimp and crayfish. Even a catfish was spotted.

The hydro power tunnel, used to obviously supply power to the mill, didn’t quite provide enough.

“The problem that Mr. Pillsbury learned very early on was that the river water … it was dependent on the climate and the weather,” said Richardson. “He very quickly decided, after buying the mill, that he needed another source of power, so he bought a steam plant and generated electricity.”

The maintenance on the tunnel was constant, however, meaning workers had to keep the turbines moving and make sure the flow of water to the drop shaft remained consistent.

The conditions were cold, damp and dark.

MORE NEWS: How Can You Tell If You're Truly Burning Out? What Can You Do About It?

Standing inside, it’s hard to believe this type of space is not unique. Even with so many underground treasures in the Twin Cities, getting a glimpse into the mystery under our feet is exceptionally rare.