The 1,800-foot-long McCook Reservoir Main Tunnel is being built in suburban Chicago to connect the future McCook Reservoir (pictured) to Chicago's Deep Tunnel system to improve water quality in area rivers and Lake Michigan and reduce flood risk for residents. The $500 million project is a key component of Chicago's Tunnel and Reservoir Plan, through which the Metropolitan Water Reclamation District of Greater Chicago collects and diverts combined sewer overflows and flood-waters from across metropolitan Chicago to temporary holding reservoirs before treatment. When completed, the 33-foot-wide tunnel will connect Chicago's Mainstream Tunnel to the planned McCook Reservoir and bolster protection of the local water supply. Photo: Black & Veatch