Officials from the Metropolitan Water Reclamation District on Monday morning said the blow out and resulting massive crater in Calumet City was caused by the pressure from severe rain.
The underground pipe burst when rain was coming down at a rate of 1.7 inches per hour, an engineer said. That caused a concrete slab to lift 15 feet underground, causing the crater that measures about 30 feet wide by 25 feet deep.
Residents who live near the crater, at Burnham Avenue and Heritage Drive in the far south Chicago suburb, described the explosion as sounding like a bomb blast. It sent sand and debris flying in all directions.
"I looked out the back door ... and all I seen was this big wave just coming," said Sharon Krueger, who lives nearby. "I went to the building, banging on doors, telling people that we were getting flooded."
No injuries were reported, but engineers conceded there have been problems with the system in recent years.
The Calumet Tunnel system was full as of 10 p.m. Sunday, officials said. The tunnel holds 620 million gallons of water and is part of the Deep Tunnel system which can hold a total of 2.3 billion gallons of water. As of midday Monday, the Mainstream Tunnel was 77 percent full, the Des Plaines Tunnel was 51 percent full, and the Kirie Tunnel was empty.