 The International Real-time Magnetic Observatory Network, with over 100 interlinked geomagnetic observatories around the world, has been recording the Earth's magnetic field since 1991.The military determines local geomagnetic field characteristics, in order to detect "anomalies" in the natural background that might be caused by a significant metallic object such as a submerged submarine.Typically, these magnetic anomaly detectors are flown in aircraft like the UK's Nimrod or towed as an instrument or an array of instruments from surface ships.