A submersible vehicle having an outer hull which defines a hull axis and appears substantially annular when viewed along the hull axis, the interior of the annulus defining a duct which is open at both ends so that when the vehicle is submerged in a liquid, the liquid floods the duct. At least part of the outer hull is swept with respect to the hull axis A buoyancy control system may be provided. Various methods of deploying and using the vehicle are described.