The tunnels are 18-feet down and just 6-feet wide. IDOT was attempting to secure the stretch of it under the Kennedy Expressway near Adams when the pavement buckled.
The tunnel network connects Chicago's downtown buildings. Construction on it began in 1899- and soon a small electric railroad freight, coal and tourists under the city.
"This system is huge. Originally it was 62 miles in length. It was all hand dug," said Bruce Moffat, Chicago Tunnel historian.
In 1992, ABC7 went into the tunnel system after contractors who were planting a pylon in the Chicago River caused a breach. Water travelled the underground grid, flooding dozens of buildings. Since then the system has been sealed-off with water tight doors and alarms.
"If we didn't fill the freight tunnels while working in this area, they could have collapsed and caused damage to the expressway itself," said Tony Albert, IDOT engineer.
IDOT engineer Tony Albert and his team knew a portion of the tunnel extended under the Kennedy from Jackson to Quincy. All along the plan was to fill the section with concrete so vibrations from the construction of a retention wall wouldn't cause a collapse. The problem on Wednesday: IDOT says a private contractor apparently pumped in too much concrete.
"There's pipes set up in there that we can actually pump the material into it to the point we remove all the air void and leave concrete in there," said Tony Albert, IDOT Engineer.
IDOT said the freight tunnel that caused the buckle Wednesday was the last remaining unfilled section underneath Chicago's expressways. But many more miles of tunnel exist underneath the Loop, sealed-off but still present 50 years after they shutdown.
IDOT isn't ready to put a price tag on Wednesday's pavement buckle and all of the overnight work done to repair it in time for the morning commute. The investigation into exactly what caused the mishap is underway.