A foundation structure for locating an object mounted on the foundation structure on a sea bed or a river bed comprises an integral buoyancy device having variable buoyancy and a rigid control surface. The integral buoyancy device is movable relative to the foundation structure to vary the position of the rigid control surface and to control the position of the center of buoyancy relative to the position of the center of gravity. This stabilizes the foundation structure during submergence and surfacing. A method for controlling the deployment and retrieval of the foundation structure is also described.