We report the discovery, in northwestern Illinois, of a geomagnetic anomaly, using hydromagnetic wave frequencies as the source spectrum. Three portable magnetometer stations with computer-compatible digital data acquisition systems were operated in a longitude array at Plano and Ashton, Illinois, and Cascade, Iowa (total separation ~200 km), in 1981--1982. Analysis of the natural geomagnetic field fluctuations in the hydromagnetic wave regime reveals that the vertical components of the detected fluctuations are essentially 180¿ out of phase between Plano/Ashton and Cascade for variations with periods ~30--120 s. The observations can be modeled in terms of a shallow (~10--20 km) north-south oriented geomagnetic anomaly of enhanced conductivity located between Ashton and Cascade, approximately parallel to the Mississippi River Valley.