A vehicle includes a submersible housing and a propulsion mechanism, which is coupled to drive the housing over an interior surface of a container in which the housing is submerged. A hollow tube is fixed to the housing and is configured to be filled with a fluid having a first specific gravity. One or more objects, which have a second specific gravity that is less than the first specific gravity, are contained within and capable of moving inside the hollow tube so as to stabilize the housing in response to changes of an angle of the interior surface over which the vehicle travels.