Hundreds of feet beneath the city, construction workers are blasting a hole for a giant  machine that will drill a massive tunnel nearly four miles long.

One day, this Waterway Protection Tunnel will help clean up the Ohio River.

During heavy rains, Louisville’s sewers overflow and the Metropolitan Sewer District has no choice but to dump excess raw sewage into the Ohio River and Beargrass Creek.

But now they have a plan. It’s actually a very simple plan: drill a massive tunnel underneath the city to store the excess sewage until the wastewater treatment plant can take care of it.

It’s such a simple concept, but actually carrying out of that idea is an immensely complex task.

“It’s not like drilling a hole in your bathroom wall to hang a picture, it takes a while,” said Todd Tharpe, an engineering geologist for contractor Black and Veatch who’s working on the project. “It’s a big monstrous machine as well with so many moving parts you wonder how it can all work together.”

Ryan Van Velzer | wfpl.org

At the bottom of the pump station dropshaft, where workers are blasting a tunnel to connect to a second dropshaft in Louisville Kentucky.

The city’s already built two dropshafts, each more than 200 feet deep and roughly 40 feet wide.

To make the tunnel, construction workers will lower pieces of the machine — it’s called a boring machine — into the dropshafts, where it will be assembled.

As it passes deep into the earth under Louisville, it will pass through a layer of sand and gravel formed during the Ice Age, Tharpe said.

“As the glaciers melted and retreated they dumped their sediment load, in other words all the boulders and rock and sand,” he said.

Beneath that, there’s an upper layer of bedrock storing marine fossils that are millions of years old.

Ryan Van Velzer | wfpl.org

Layers of rock inside the pump station dropshaft 200 feet underground in Louisville, Kentucky.

Tharpe says these fossils were originally deposited around the equator. That’s where Louisville actually was at the time, before the landmass shifted and moved the North American continent to where it is now.

“You see branch corals, and you see various types of mollusks and you see remnants of sponges and sea creatures,” he said.

Of course, you can’t actually see those fossils in the giant pump shaft, but that’s where they are, behind the layer of chemicals sprayed on the walls to keep the rocks in place.

At the bottom of the pump shaft, it’s damp and rocky. There’s a massive hole in the wall where workers are blasting rock to connect to another dropshaft a few hundred feet away.

It’s at this layer of 350 million-year-old limestone that the city plans to drill a nearly four-mile long tunnel underneath the Louisville.

The boring machine will move 90 to a 100 feet on a good day, Tharpe said.

MSD expects it will take three years to complete the construction on the tunnel. When it’s finished, they say it will capture 98 percent of sewage and stormwater overflows.

Ryan Van Velzer is WFPL's Energy and Environment Reporter.