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Workers Putting Finishing Touches On Thornton Reservoir

(CBS) -- For perhaps the first, and last time, CBS 2'scameras went inside a place that could be key in keeping your home and neighborhood free of floodwaters.

The numbers are staggering and the engineering nothing short of miraculous. CBS 2's Derrick Blakley takes us to an old place with a newfound purpose.

The Thornton Reservoir, a century-old limestone quarry is about to become a giant holding tank.

"It's going to hold 7.9 billion gallons of combined stormwater and wastewater to relieve the system when we have those heavy rainstorms," said Mariyana Spyropoulos, Commissioner of the Metropolitan Water Reclamation District.

The vast open pit is roughly a square half mile and more than 30 stories deep. You've probably seen it from I-80, but you haven't seen it full of water, which could happen, once its connected to the Deep Tunnel.

"When that's full, and it has happened before, it will overflow into the reservoir," said Spyropoulos.

The tunnel lies 300 feet below ground, its 30 feet in diameter and inside it's never more than 55 degrees.

Deep inside, work crews are preparing to install the gates that will control the flow to the reservoir.

"With the gates we will be able to control the flow of water going into the reservoir and the flow of water going out of the reservoir to the treatment plant," said MWRD engineer Augustine Eyinie.

Thornton will be the second reservoir to come on-line. The Majewski Reservoir, near O'Hare, is already open.

The last, the McCook Reservoir, won't be totally finished until 2029.

With Thornton open, 14 south suburban communities and parts of Chicago's South Side should see a lot less flooding.

"From our calculations, we're expecting at least 40 million dollars in damage to be mitigated every year," Spyropoulos said.

The Thornton Reservoir is expected to be ready for duty in just a few weeks.

Another benefit: It's expected to reduce the need for opening that separate the Chicago River system from Lake Michigan during storm events and that will keep Lake Michigan water cleaner.

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