U-boats during WW2 used many decoys to foil enemy sonar and periscopes and one of the most interesting was a decoy called BOLD. BOLD was a chemical cylinder 3.9" in diameter that once launched from the U-boat absorbed sea water that mixed with calcium hydride to form a screen of hydrogen bubbles. A valve opened and closed so that the cylinder stayed around 98 ft depth. This usually bought the U-boat 20-25 minutes to escape or dive. BOLD progressed in numbers 1-5, the last in 1945 allowing a depth of 695 ft. Two other decoys were often used in conjunction- Sieglinde which provided a false acoustic signature than matched a U-boat and Siegmunde that produced loud explosions to "black-out" sonar.