The Thornton Reservoir is the final stage of the TARP tunnel project's Calumet branch. When the former limestone quarry fills, it will add 7.9 billion more gallons of capacity to Chicago’s stormwater runoff system.

The Calumet Tunnel System opens into the Thornton Reservoir, a re-used limestone quarry.

The reservoir will hold 7.9 billion gallons of a mixture of storm water and sewage.

To minimize the odor of the sewage, seven solar powered aerators will float on top of the reservoir.

A closer look at an aerator that will float on the reservoir and circulate the top 5-10 feet of water.

Interstate 294/80 runs along the edge of the reservoir.

The service road into the reservoir will soon be underwater.

Temporary offices and heavy machinery are dwarfed by the reservoir walls.

This board keeps track of which workers are currently in the tunnel.

A worker walks out of the tunnel for lunch break.

Supplies and debris left by workers near the entrance to the tunnel.

Workers return to the tunnel after lunch.

The area where gates will control the flow of water into the reservoir was one of the final work zones in the project.

Three workers build the gate mechanism in the tunnel.

Light bends along the curve of the tunnel.

Reflections in water collected on the bottom of the tunnel.

Heavy machinery backs out of the tunnel.