Abstract: A borehole reamer which may be deployed to a desired depth in an existing uncased borehole or below existing casing and used to substantially expand the diameter of the borehole. The diameter may be expanded by an amount that ranges from a fraction to a multiple of the original diameter of the borehole. In this fashion, geological formations containing oil, gas or groundwater which were previously uneconomical to produce may be worked into a condition of economical production. In a preferred embodiment, the borehole reamer is formed as an elongated assembly having an outside diameter which is sized to fit within a diameter of a conventional borehole. The assembly is preferably attached to drill pipe and lowered into a borehole to an appropriate depth corresponding to a pay zone of interest. A lower portion of the assembly, which is located at the deeper depth in the borehole, is constructed in the form of a bellows which functions as a pilot/anchor for the assembly.