Abstract: An energy conversion apparatus for converting compressed gas to shaft power includes a housing divided between first and second chambers in fluid communication with each other and adapted to maintain a water column in the second chamber when the first chamber is pressurized with the compressed gas. The air filled first chamber houses a gravity drive system which harnesses energy created by the gravitational forces acting on a plurality of downwardly moving cylindrical containers grasped by receiving arms disposed on at least one gravity drive chain in the first chamber. The second chamber houses a buoyancy drive system which harnesses energy created by buoyant forces acting on a plurality of upwardly moving cylindrical containers grasped by receiving arms disposed on at least one buoyancy drive chain in the second chamber. An external drive shaft is driven by a coupling means which combines the energy created by the buoyancy and gravity drive systems.