On 28th September 1942 Adolf Hitler and senior Kriegsmarine officers engaged in conference in Berlin, one of the prime reasons being Hitler's desire for a "decoy torpedo" to simulate the destruction of U-boat. His original opinion was that special torpedoes carrying oil and various items of wreckage could be carried and launched in the event of a prolonged hunt by enemy destroyers. However the loss of an operational torpedo tube for the carrying of such a device was unacceptable to U-boat officers.
The compromise that was reached resulted in probably the most effective of the German anti sonar counter-measures. Named Bold (short for Kobold meaning "deceiving spirit" or "goblin") this device comprised a 15cm diameter capsule filled with 370grams of calcium and zinc that was expelled from the stern compartment of a U-boat through a purpose built ejector (named the Pillenwerfer - pill-thrower - by crewmen). The calcium-zinc compound was packed within a wire mesh bag, in turn stored inside an aluminium canister. Sealed within a waterproof outer layer there was a hydrostatic valve that controlled the entry of seawater into the canister. Upon contact with sea water the compound within the capsule produced hydrogen gas resulting in a large mass of bubbles that resembled on ASDIC the echo produced by a submarine contact. The hydrostatic valve regulated the entry of seawater to such an extent that the Bold capsule could continue to emit bubbles for up to 25 minutes.
Two other canisters were developed to be launched by the U-boat's Pillenwerfer. The first was a miniature torpedo that carried no warhead but only an electric motor that would circle at a given depth and simulate the noise created by a U-boat's electric drive before its small battery was exhausted. The second more closely mirrored Hitler's original intention and contained oil which rose to the surface creating a small oil slick for attackers to see.