Commonwealth Edison recently drilled three engineering test holes in northwestern Illinois. The three holes (UPH 1, 2, and 3) were 0.6, 1.6, and 1.6 km deep, respectively, and the latter two penetrated nearly 1 km of Precambrian granitic basement. In 1980, continuous core from all three holes and access to UPH 3 were made available for scientific investigations through the Illinois Deep Hole Project. The drill hole and cores provided an excellent opportunity for ''piggyback'' scientific investigations because (1) the basement geology of the region was poorly known, (2) a large vertical depth of continuous core was available, and (3) one of the holes was available for testing where corresponding core is available. The principal study of these holes was an integrated investigation of surface and in-hole experiments, plus detailed petrologic, geochemical, and physical property studies of the core samples. The combined results of these studies contributed significantly to our understanding of the regional geology of the Precambrian basement, the regional geophysics and tectonics, and the correlation between geophysical well logging and the true character of the rocks penetrated. Furthermore, this project has shown that substantial scientific benefit can be obtained from continental drilling ''holes of opportunity'' if members of the scientific community are able to respond in a timely and coordinated fashion.