A network of underwater microphones called hydrophones, used by militaries to identify submarines and warships, operate in oceans around the world. Two of them were positioned in the Indian Ocean when the Malaysia Airlines Boeing 777 jet vanished with 239 people aboard while flying from Kuala Lumpur to Beijing on March 8, 2014.

One known as HA01 was off Cape Leeuwin on the Western Australia coast, while the second known as HA08 was at a US military base in Diego Garcia, just south of the equator in the heart of the Indian Ocean. But the information picked up by the device in the US base was distorted - and 25 minutes is missing.

'[The signals] were distorted by noise which is believed to have been caused by a military exercise in that side of the ocean.

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