Core drilling is a system in which a core sample (cylindrical) is drilled and extracted in the condition in which it is found, i.e. in situ. So here wire line coring system is used, which means that a (hollow) drill bit impregnated with diamonds is used, which cuts until the core tube is full. Drilling mud/cutting oil cools/lubricates the drill bit and serves as a medium to lift the cuttings to the surface.
The core drilling method gives the geologist the ability to analyse the sample both by eye and using more advanced methods.
Because the samples are placed piece by piece in core boxes and carefully marked, a complete picture of the rock layers emerges.