Core:

To deepen the wellbore by way of collecting a cylindrical sample.

A core bit is used to accomplish this, in conjunction with a core barrel and core catcher.

The bit is usually a drag bit fitted with either pdc or natural diamond cutting structures, but the core bit is unusual in that it has a hole in its center.

This allows the bit to drill around a central cylinder of rock, which is taken in through the bit and into the core barrel.

The core barrel itself may be thought of as a special storage chamber for holding the rock core.

The core catcher serves to grip the bottom of the core and, as tension is applied to the drillstring, the rock under the core breaks away from the undrilled formation below it.

The core catcher also retains the core so that it does not fall out the bottom of the drillstring, which is open in the middle at that point.