Updated 04/18/13 – 12:26 p.m.

CHICAGO (CBS) — The huge amount of rain over the past 24 hours has filled the large underground flood control system known as Deep Tunnel.

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The Metropolitan Water Reclamation District’s Tunnel and Reservoir Project (TARP), better known as “Deep Tunnel,” was designed to absorb and store 2.3 billion gallons of excess storm water before it reaches local waterways.

While the Tunnel has been filled to capacity before, it is rare. It’s happened only three or four times in the past four years. The last time was in 2011.

MWRD supervising civil engineer Ed Staudacher said the Chicago area hasn’t had a storm like this since July 2011.

“For this size of storm, it’s not unusual, but we don’t see this size of a storm that often,” he said. “Once the system … the tunnel is full, it reverts back to the way it was before the tunnel is there, and it overflows into the waterways.”

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The locks at Wilmette, Navy Pier and Calumet Harbor have been opened to pour storm water into Lake Michigan. That’s what the Deep Tunnel is designed to prevent, but it also keeps local rivers and canals from overflowing even more than they would if the locks were closed.

“The Chicago area waterways are mostly manmade, man-altered waterways, and we control the elevations within those waterways … the best we can,” Staudacher said.

He said he didn’t know how long it would be until the Deep Tunnel would begin to empty.

The $4 billion Deep Tunnel was designed to handle to water pollution and sewer backup problems in 52 municipalities in Cook County.

The system has 109 miles of tunnels, nine to 35 feet in diameter, 350 feet below ground.

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The tunnels collect storm water and send it to to quarries and other reservoirs until the water can be treated.