Chicago’s ambitious Tunnel and Reservoir Plan (TARP) isn't just one of the biggest megaprojects in America – it's also one of the largest in the world.

The huge civil engineering project centers on a system of deep, large-diameter tunnels and vast reservoirs, designed to reduce flooding and clean up the local waterways. TARP collects and stores combined stormwater and sewage that would otherwise overflow from sewers into waterways when it rains. This water is then pumped from TARP to plants, where it is cleaned and released.

The project was first commissioned back in the 1970s with completion expected in the 1980s. However, construction of the sewer system is still ongoing and the completion date has now been delayed to 2029, meaning the project will have taken well over 50 years to finish.