"Aphrodite
Aphrodite was a decoy balloon. Filled with hydrogen, it floated a few meters above the water surface, anchored to a floating plate. Aluminium foils attached to the balloon gave a strong radar echo, similar to that of an U-boat on British radar screens. "

My error. I mis-remembered reading that the aphrodite balloon floated on the surface. (smacking head)

In that case. Then it is probable that there would be gas tanks in the conning tower.