Abstract Low-gradient streams and generally flat topography have always presented a problem of flooding within the Chicago area. As settlement expanded, the flooding produced greater and more serious results. As early as 1816, the city's leaders began major engineering work to solve these problems. The latest efforts toward a solution currently include a system of over 100 miles (161 km) of large-diameter tunnels in the Chicago area bedrock. These tunnels will intercept overflow from combined sanitary and stormwater systems and convey it to temporary storage reservoirs prior to its being pumped to sewage treatment facilities. Geologic investigations made during a study for the tunnel sites included test drilling and coring, geophysical logging of boreholes, laboratory testing of samples, seismic surveying, and testing for groundwater. The drilling, coring, and logging furnished data that have been of considerable help in mapping and describing the individual units of Silurian and Ordovician strata of the area. The seismic survey indicated numerous closed depressions on the surface of the bedrock and also sug-gested that several faults are present with displacements of 10 to 50 feet (3 to 15 m).