Abstract: An underwater mine warfare countermeasure system is disclosed in which an air cushion vehicle carries an acoustic transmitter for sending out an acoustic wave pattern capable of exploding mines having acoustic responsive detonators. The vehicle includes a transmitter retraction apparatus for raising and lowering the transmitter between a storage position located just beneath the vehicle and an operative position spaced remotely from the vehicle, and also from raising the transmitter through an opening in the vehicle for placing it in an access position on the vehicle. The vehicle also includes an air seal or pressure boundary assembly which normally forms an air-tight cover for the aforementioned opening while still permitting the transmitter to be deployed to the operative position and retracted to the storage position while the vehicle is on cushion, but which opens to permit removal of the transmitter to the access position when the vehicle is off-cushion.