A single-blade underreamer for forming a cavity within a well bore includes a housing rotatably disposed within the well bore, and a stabilizer coupled to the housing. The stabilizer is operable to stabilize the housing within the well bore during formation of the cavity. The underreamer also includes a single cutter pivotally coupled to the housing, and a piston slidably disposed within the housing and adapted to engage the cutter. A downwardly disposed force applied to the piston is operable slide the piston relative to the housing to correspondingly extend the cutter outwardly relative to the housing from a retracted position to form the cavity during rotation of the housing.