Fake Japanese periscope
Australian War Memorial image of a dummy (fake) Japanese submarine periscope. The ruse was simple yet effective. A bamboo pole was painted black and a mirror fixed to the top to reflect sunlight and attract the attention of patrolling aircraft. The pole was weighted down with glass Saki bottles filled with diesel oil, causing the 'periscope' to float in a vertical attitude; for all intents and purposes looking like a real periscope from a submerged Japanese submarine. If attacked by allied aircraft, the percussion shock from bombs will shatter the glass bottles, causing diesel oil to spread across the water. This would deceive the attacking aircraft into thinking it had scored a hit and 'sunk' the submarine which was long gone.