An underreamer for well drilling comprises pivotally extendible roller cutters that extend slightly beyond the diameter of the pilot drill bit but are retained in their retracted position by the casing. Upon encountering an obstruction, the underreamer is pushed out of the casing, allowing the outer edges of the cutters to contact the obstruction. Continued downward pressure of the drill stem causes the roller arms to pivot outward to a fully extended position in which the cutters enlarge the hole to a diameter larger than the casing by rotation of the drilling stem. The advance of the casing relative to the underreamer forces the cutters to retract into the casing.