Short answers, pulled from the story.
Anti-submarine warfare (ASW) is the branch of underwater warfare that uses surface warships, aircraft, submarines, and other platforms to find, track, and destroy enemy submarines. It is considered of significant strategic importance particularly because of unrestricted submarine warfare and the introduction of submarine-launched ballistic missiles, which greatly increased the destructive potential of submarines.
The first recorded sinking of a submarine by depth charge occurred on the 22nd of March 1916, when U-68 was destroyed by a Q-ship off County Kerry, Ireland. The Type D depth charge, which carried a 300-lb charge of TNT, had only entered operational trials in June 1915.
The introduction of escorted convoys reduced shipping losses in the German war zone around the British Isles from 25 percent to less than 1 percent. Historian Paul E. Fontenoy described the convoy system as having defeated the German submarine campaign outright.
Codebreakers at Bletchley Park in England broke German Naval Enigma codes, a program whose intelligence product was called Ultra. This allowed Allied commanders to track U-boat packs and reroute convoys away from them. When the Germans added a fourth rotor to their Enigma machines in 1943, convoy losses rose significantly until the new configuration was solved.
American torpedoes in the early Pacific War routinely failed to detonate on impact, ran too deep, or went off course. US Vice Admiral Charles A. Lockwood ran his own tests to prove the weapons' unreliability after the ordnance department initially ignored his complaints. Once the torpedoes were fixed and aggressive commanders were installed, US submarines accounted for nearly half of the Japanese merchant fleet.
SOSUS is a system of bottom-mounted hydrophones deployed by the United States in the GIUK gap and other strategically important ocean passages. It uses land-based processing to monitor submarines crossing those chokepoints. Some SOSUS arrays have since been turned over to civilian use for marine research.