A multiple stage buoyancy changing system, or variable buoyancy device, for an underwater vehicle. The multiple stage buoyancy changing system comprises: a pressure hull containing a flexibly-sized internal fluid reservoir; a flexibly-sized external fluid reservoir attached to the pressure hull and connected to the internal reservoir; a system of devices and channels configured to move fluid between the internal fluid reservoir and the external fluid reservoir to change a displaced volume of the vehicle. Each stage of the variable buoyancy device can be optimized for maximum energy efficiency while changing the vehicle's displaced volume within an ambient pressure range. A control system for the variable buoyancy device engages different stages depending on ambient external pressure such that maximum energy efficiency is achieved over a large range of pressures/depths.