(1) A hole in the rig floor 30 to 35 feet deep. Lined with casing that projects above the floor into which the kelly is placed when hoisting operations are in progress. (2) A hole of a diameter smaller than the main hole. It is drilled in the bottom of the main hole.

a hole (as in the wall of a building) made by rats

Shallow bore under the derrick substructure in which the kelly joint is temporarily set while making a connection.

A hole in the drilling floor in which the kelly joint is kept when not in use.

Extra hole drilled at the end of the well (beyond the last zone of interest) to ensure that the zone of interest can be fully evaluated. The logging tool string may be as much as 120 ft [36.5 m] in length, so the rathole allows tools at the top of the logging string to reach and measure the deepest zone of interest. In addition, there is usually a small amount of extra hole drilled to allow for junk, hole fill-in and other conditions that may reduce the effective depth of the well prior to running logging tools.