Urban sprawl around the City of Chicago has increased the demand for construction aggregates in northeastern Illinois. Limited surface land available for quarry expansion and a cumbersome surface permitting process have led to an increase in underground limestone and dolomite mines in the Chicago area. Since 1991, the Conco Mine in North Aurora has been mining limestone and dolomite underground. Previously owned by Conco-Western and now owned by Lafarge North America, the Conco Mine continues to prove that underground aggregate mining around a large metropolis can be economical and successful. An exciting discovery during expansion of the Conco Mine was the presence of massive solution cavities lined with large specimen-quality minerals, such as calcite, pyrite, and marcasite. The scientific importance of these minerals has been recognized, and the mining company has allowed the author to extract specimens. Research has been ongoing to interpret the diagenetic fluids responsible for carbonate diagenesis of the limestone host rock and the precipitation of world-class mineral specimens.