technique used in underground or undersea exploration and prospecting. A core sample is a roughly cylindrical piece of subsurface material removed by a special drill and brought to the surface for examination. Such a sample is needed to ascertain bulk properties of underground rock, such as its porosity and permeability, or to investigate the peculiar features of a given zone of strata (e.g., to compare strata at a given level with those known to bear oil or gas).

A further purpose of employing coring devices is to recover samples of the several layers of fine-grained deposits on the seafloor in such a way as to preserve the depositional sequence. By studying the contained mineral grains, microfossils, and interstitial water (water in the pore spaces), scientists have been able to infer the depositional history and past oceanic events. In an additional application of core sampling, polar ice sheets have been penetrated to secure information about the age and rate of accumulation of the ice.

Coring tools are long metal cylinders. These may be forced beneath the surface, or sediments may be drawn into them by means of suction.