A submergible vehicle capable of untethered, fixed depth operation is provided with a rigid walled buoyancy chamber (40) adapted for accommodation of varying volumes of water and buoyancy gas. The chamber is provided with a valving system to adjust the volumes of gas and water within the chamber and therefore control the operating depth of the vehicle. Gas is provided by a high pressure source (10) thereof, the gas also serving to positively pressurize a hull (15) of the vehicle to accommodate the compressive loading thereof by water exteriorly of the vehicle.
