LOUISVILLE, Ky. (WDRB) -- To some, it might have appeared to be complex construction project — albeit an expensive one. 

On Monday, crews lowered a concrete slab over the mouth of a giant tunnel, or shaft, at the corner of North 12th and Rowan streets in Louisville's Portland neighborhood. There would be six slabs total, weighing a total of 220 tons and forming a cap over the shaft that stretches 48 feet in diameter. 

The capping of the shaft is the latest — and one of the last — milestones in the Metropolitan Sewer District's $200 million Waterway Protection Tunnel. But to David Johnson, MSD's chief engineer, Monday was about more than just checking a box.

It was about penance.

"This is gonna prepare us for the future," he said. "But it's making up for the sins of the past."