Chicagoans can rest easy. In the event that heavy rains overload the city’s sewers, the excess water will no longer overflow into the environment, polluting ecosystems and sometimes filling basements with toxic floodwater.

Instead, it will go here:

This is the Thornton Composite Reservoir, also known as the “Grand Canyon of the South Suburbs.” It’s ostensibly a 7.9 billion-gallon bathtub commandeered from a quarry, designed to hold rain and sewage overflow until it can be piped elsewhere for treatment.

Over a rainy Thanksgiving weekend, the reservoir took on water for the first time, filling with 400 million liters — mostly stormwater, tinged with a soupçon of sewage.

At 17 feet deep, the reservoir was at only 5 percent of capacity. That intake pipe has a diameter of 30 feet.

The problem is one that a lot of cities face. In the past, sewage and storm water systems were designed as one. When heavy rains push the systems beyond capacity, the excess is allowed to overflow into the environment — which, in the case of Chicagoland, includes Lake Michigan.

This is a pretty gross and unhealthy situation, and one that Chicago is spending $3.8 billion to fix. The Metropolitan Water Reclamation District of Greater Chicago’s full initiative involves building more than 100 miles of tunnel and three reservoirs for the city.

The Thornton Composite Reservoir will ensure that 182,000 homes and businesses don’t have to suffer stinky floods in the future.

Photos via MWRD Public Affairs / YouTube