CHICAGO (CBS) — Local federal and environmental officials on Tuesday heralded the completion of a new south suburban reservoir that will provide major flood protection to the area.

The Metropolitan Water Reclamation District of Greater Chicago cut the ribbon Tuesday morning at the Thornton Reservoir, which can hold 7.9 billion gallons of tormwater and sewage. The project has been decades in the making, and is meant to keep floodwaters and sewer overflow at bay in 13 suburbs and on the South Side of Chicago.

The $429 million Thornton Reservoir is a section of the Thornton Quarry, which has been used to mine limestone for nearly two centuries. The MWRD has converted the north lobe of the quarry into a gigantic holding tank, measuring about a square half mile, and more than 30 stories deep. The north lobe of the quarry has been sealed with what MWRD President Mariyana described as miles and miles of grout.

“152 billion pounds of 400-million-year-old dolemite limestone has been removed to create this reservoir. It’s going to hold 7.9 billion gallons of water, benefit 14 communities, and minimize or mitigate $40 million in flooding damage,” she said.

Water will flow to the reservoir through a massive tunnel 300 feet below ground, and 30 feet in diameter. It’s connected to the Deep Tunnel system, a 100-mile network of tunnels under the Chicago area used to capture and hold stormwater and sewage for treatment at water reclamation plants.

For U.S. Sen. Dick Durbin (D-Ill.), the site alone was impressive, and the mission necessary.

“It’s great to be with you today, and to marvel at Chicago’s Grand Canyon in this magnificent reservoir,” he said.

The reservoir should help improve water quality in the area, by reducing the need to release sewage into Lake Michigan when heavy storms overwhelm storm sewers by opening the locks separating the lake from the Chicago River. It should also help prevent flooding to nearly 200,000 homes and businesses during heavy rains.

Thornton is the second MWRD reservoir to come online. The Gloria Alitto Majewski reservoir near O’Hare International Airport opened in 1998, and holds 350 million gallons of water.

The McCook Reservoir, which is still under construction, will hold about 10 billion gallons when complete. The first phase, with a capacity of 3.5 billion gallons, should be completed by 2017; and the second phase, with a capacity of 6.5 billion gallons, should be completed by 2029.