The Geological Records Unit (GRU) is the state repository for drilling records as mandated by statute. The collections include records for more than 500,000 oil and gas wells, water wells, engineering borings, and miscellaneous test holes drilled in Illinois. Many regions of the state lack abundant outcrops and road and stream cuts due to relatively flat topography. Drill hole records provide the most abundant data available for geologic interpretation. These data are useful for environmental assessment, mineral resource evaluation and groundwater resource evaluation and protection. GRU is the gateway to Illinois geology.