The Thornton Quarry, which has been described as the Grand Canyon of the Midwest, nearly filled up with rainwater after this weekend's storms. The Thornton Composite Reservoir, which is just off Interstate 80 west of the state line, was 94% full Monday, the highest level since part of it was turned from a quarry into a reservoir in 2015, according to the Metropolitan Water Reclamation District of Greater Chicago. The high-water mark was previously 54.5% full in 2019.

More than 7.3 billion gallons of water were captured in the reservoir on the north side of the quarry. It has a capacity of 7.9 billion gallons.

"The reservoir is working as designed. We are actively dewatering the reservoir now to provide room for additional stormwater," Public and Intergovernmental Affairs Officer Allison Fore said. "If the reservoir reaches capacity, sewers can still overflow to the river as they did before TARP was built."

About 3.77 inches of rain was measured after the storm by a Metropolitan Water Reclamation District of Greater Chicago rain gauge at the Calumet Water Reclamation Point. The latest storm added to an already wet year.

More than 13 billion gallons of stormwater have been captured in deep tunnels and reservoirs after multiple rounds of storms, according to the Metropolitan Reclamation District.

The Thornton Reservoir was 94% full after the weekend's storms.

The reservoir system was developed to protect Lake Michigan, the Chicago metropolitan area's main water supply, by keeping pollution from flooding in. Heavy rains can cause sewers to overflow, spilling raw sewage into waterways. The Thornton Reservoir has completely eliminated any combined sewer overflows in the south suburbs since 2020.

Large enough to fit six Soldier Fields inside it, the Thornton Reservoir collects rainwater and sewage before sending it in a controlled and measured fashion to the Calumet Water Reclamation Plant five miles north, where it's treated and released into the Little Calumet River. It's big enough to hold the equivalent of 144 million rain barrels and could circle the entire Earth 3.64 times if laid end to end.

Stretching 450 feet deep, the Thornton Quarry was one of the largest aggregate quarries in the world, supplying materials for countless construction projects. As a reservoir, it now protects more than half a million people in more than a dozen suburbs just across the state line.