section 140 of the hull 122 and water 164 contained in the lower part of the interior of the second section 140 of the hull 122 opening the valve of the valve assembly 146 allows air 162 to vent from the hull 122 through the opening 144 and the outlet 145 as shown by the arrows in fig6 which permits more water 164 to flood into the hull 122 through the openings 142 thereby reducing the buoyancy of the ballast tank 102 and causing the depth of the auv 100 to increase to increase buoyancy and decrease the depth of the auv 100 the valve of the valve assembly 146 is closed and the valve of the valve assembly 154 is opened to introduce high pressure gas into the hull 122 the high pressure gas forces water 164 out of the openings 142 in the hull 122 as shown by the arrows in fig6 which increases the amount of air 162 in the hull 122 and increases the buoyancy of the ballast tank 102 the ballast tank 102 is a structure that is separate from the auv 100 and is not a part or sub part of the auv 100 as a result the displaced volume of the auv 100 remains the same before and after release of the ballast tank 102 from the auv 100 the auv 100 and the ballast tank 102 are releasably attached in any suitable manner for example the auv 100 and the ballast tank 102 can be releasably attached in a manner similar to the attachment described above for the auv 10 and the payload 12 shown in fig1 3.