An underreamer for forming a cavity from within a well bore includes a housing adapted to be disposed within the well bore. The underreamer includes an actuator partially slidably positioned in the housing. The actuator comprises a first portion and a second portion. A cross-sectional area of the second...http://www.google.com/patents/US6976547?utm_source=gb-gplus-sharePatent US6976547 - Actuator underreamer
