1. n. 
A dug-out area, possibly lined with wood, cement or very large diameter (6 ft [1.8 m]) thin-wall pipe, located below the rig. The cellar serves as a cavity in which the casing spool and casinghead reside. The depth of the cellar is such that the master valve of the Christmas tree are easy to reach from ground level. On smaller rigs, the cellar also serves as the place where the lower part of the BOP stack resides, which reduces the rig height necessary to clear the BOP stack on the top. Prior to setting surface casing, the cellar also takes mud returns from the well, which are pumped back to the surface mud equipment.