The invention relates to an auger for making a borehole that presents in its inside wall a groove that is substantially helical, said auger comprising: a hollow core provided with an outer helical blade; a movable cutter gouge suitable for taking up an active position in which the gouge projects outside the volume defined by the periphery of the blade, and a retracted position in which it is placed inside said volume; and means for displacing said gouge from its retracted position to its active position and comprising an additional member mounted to move in said core in translation and/or in rotation, and placed at the bottom end of said core, together with control means for causing said gouge to be displaced from its retracted position to its active position in response to said additional member moving relative to at least the bottom portion of said hollow core.