The Allied Submarine Detection Investigation Committee, or ASDIC, was a British organization charged with investigating and countering the German U-Boat threat during and after World War 1. The British suffered heavy losses due to U-Boat technology, and had to quickly develop a countermeasure.
The ASDIC submarine detection system was developed between World War 1 and 2 as an evolution of the hydrophone. Comparable to today's active SONAR, ASDIC functioned by sending signals out into the water and detecting echoes. The array consisted of a dome beneath the ship that rotated, sending highly directional sounds and listening for replies. Arrays were hand-cranked and replys were interpreted through headphones only. It was possible, though uncommon, to use the array in a passive mode, as well, wherein it only listened for underwater noises, such as those generated by a U-Boat propeller.
Being a hastily developed and untested technology, it encountered many technical problems. Signals were distorted by differences in temperature and salinity of the water, the speed of both the detecting ship and the submarine, by other noises in the surrounding water, and by other ships in the same convoy employing ASDIC. It was only effective at speeds of less than 18 knots, and was not effective in poor weather. All of these factors made using the technology a bit of a black art amongst Navy personnel, and few could use the technology to its fullest.
It was, however, remarkably effective when used properly. ASDIC technology, along with depth charges and aerial surveillance, was a cornerstone of the Royal Navy anti-submarine effort. The technology was used, before the end of World War 2, to equip all Allied navies, and played a large role in the defence of North America. New classes of anti-submarine ship (Corvette, particularly) employed the technology extensively, and it was used in fixed emplacements around harbours all over the world.
There is a restored Corvette class ship permanently on display in Halifax Harbour, if you want the opportunity to see an ASDIC system in person.