Admiral Lord Charles Beresford, circa 1900, submarines are "under-handed, under-water, and damned un-English"https://upload.wikimedia.org/wikipedia/commons/thumb/f/f1/Pillenwerfer.jpg/220px-Pillenwerfer.jpg (https://en.wikipedia.org/wiki/File:Pillenwerfer.jpg)BOLD:These were a pellet of calcium hydride (https://en.wikipedia.org/wiki/Calcium_hydride) in a simple metal container. On contact with sea water, the calcium hydride decomposed to produce a trail of hydrogen gas bubbles that acted as a bubble curtain (https://en.wikipedia.org/wiki/Bubble_curtain) and reflected ASDIC (https://en.wikipedia.org/wiki/ASDIC) impulses to produce a false target. The container trapped hydrogen and floated, with a crude spring valve to maintain buoyancy to keep it at a constant depth.
Later decoys, such as Sieglinde (https://en.wikipedia.org/wiki/Sieglinde_(decoy)) : Similar to BOLD, this device was a slight variation where it was designed to resemble a submerged U-boat moving at 6 knots. The Sieglinde was installed in chambers on the sides of the U-boat. It could be ejected to a considerable distance from the boat when attempting to hide from a seeker's sonar (https://en.wikipedia.org/wiki/Sonar) equipment. The Sieglinde was powered by electric motors, allowing it to move at 6 knots (11 km/h), and to periodically ascend or dive, thus imitating the sonar return of an actual submarine. This allowed the real U-boat to slip away quietly from pursuing ships. It was typically used in combination with Pillenwerfer (or BOLD (https://en.wikipedia.org/wiki/Bold_(decoy))) decoys
Siegmund: This was an anti-sonar device which emitted a series of deafening explosions and were intended to blackout the enemy’s listening gear. The U-boat would make its getaway by altering course or running at high speed during this short period. https://upload.wikimedia.org/wikipedia/commons/thumb/0/0a/Bathythermograph.jpg/220px-Bathythermograph.jpg (https://en.wikipedia.org/wiki/File:Bathythermograph.jpg)<allied version: A bathythermograph (called the same thing on German boats) mounted on submarines: By monitoring variances, or lack of variances, in underwater temperature or pressure layers, while submerged, the submarine (https://en.wikipedia.org/wiki/Submarine) commander could adjust and compensate for temperature layers that could affect sonar (https://en.wikipedia.org/wiki/Sonar) accuracy. This was especially important when firing torpedoes at a target based strictly on a sonar fix.
More importantly, when the submarine was under attack by a surface vessel using sonar, the information from the bathothermograph allowed the submarine commander to seek thermoclines (https://en.wikipedia.org/wiki/Thermocline), which are colder layers of water, that would distort the pinging from the surface vessel's sonar, allowing the submarine under attack to "disguise" its actual position and to escape depth charge damage and eventually to escape from the surface vessel. AS Sailor Steve has pointed out previously the thermalcline layer does not hide the submerged U-boat; but like a spear fisherman looking down at a target fish through the surface or a straw in a water glass, the actual position is altered slightly..enough for an able commander to skirt disaster...but not for long! Of some interest: http://harpgamer.com/harpforum/index.php?/topic/3527-tactics-101-anti-submarine-warfare-asw-part-2/ (http://harpgamer.com/harpforum/index.php?/topic/3527-tactics-101-anti-submarine-warfare-asw-part-2/)Things have gotten worse since WWII; even Blue whales are changing their song pitches. https://student.societyforscience.org/article/underwater-racket (https://student.societyforscience.org/article/underwater-racket)