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.