equipment Optical detection Initial methods involved making visual contact with the submarine, and remains an important method of target confirmation. This may now be supplemented by thermal techniques. However, the low "indiscretion rate" of modern submarines means that optical detection is now less successful. Radio intercept The use of the "wolf pack" by submarines in both the first and second World Wars allowed interception of radio signals. Though these were encrypted, they were broken by the British at "Room 40" in the First World War and by Bletchley Park during the second. This allowed convoys to be diverted and hunter-killer groups to be targeted on the pack. Submarines now transmit using methods that are less susceptible to intercept. In World War II, high frequency direction finding (HF/DF or "Huff-duff) was used by Allied escort vessels to detect submarines making position or sighting reports. The direction finding technology was not thought feasible for installation on a warship by the Kriegsmarine and they attributed successes of Huff-duff to radar technology of a performance that did not exist at the time. The standard escort tactic was to steer at speed in the direction of an HF/DF bearing until the target submarine was sighted (often visually, but sometimes on radar) and open fire before the submarine dived. If the submarine dived before being shelled, it could be hunted on sonar. Radar Radar was a prime tool in World War II for locating surfaced submarines. After development of the snorkel, and then of nuclear-powered submarines, submarines rarely surfaced outside their home port, rendering direct radar detection largely useless. However, it is possible that radar can detect the surface effects produced by a submarine. Sonar Since World War II, sonar has emerged as the primary method of underwater detection of submarines. The most effective type has varied between active and passive, depending on the countermeasures taken by the submarine. Its versatility has increased with the development of air-dropped sonobuoys, which relay sonar signals to overhead aircraft, dipping sonar from helicopters and fixed long range systems. Magnetic anomaly detection A magnetic anomaly detector (MAD) is an electronic magnetometer designed to measure magnetic field variations caused by large metal objects, such as the steel hull of a submarine. Before the development of sonar buoys, MAD gear was often installed in aircraft to pick up shallow-submerged submarines. It is still used today. Other non-acoustic methods Submarine detector loops were