109-mile-long tunnel maze hidden beneath city that could save it from climate change Deep down 90 metres under the streets of Chicago, its towering skyscrapers and greasy deep dish pizza joints is a secret underground tunnel network that sprawls over 100 miles.
However, digging down under the city would cost a huge amount, so the authorities decided to raise the city out of the mud by two metres, adding some above-ground sewage pipes and filling in the gaps with dirt.
All the newly laid sewers spread across the city flowed directly into the waterways, which also flowed into Lake Michigan — the source of the city's drinking water.
Storms would fill the streets with mud, and pools of standing water would dominate patches of the streets across the entire city.
The Tunnel and Reservoir Plan – also known as the Deep Tunnel system — is one of the largest civil engineering projects in the world, where construction has been underway since the 1970s.
