The Titanic Sub: How significant is the banging?

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Five people are feared trapped inside a missing submersible which was attempting to travel deep into the Atlantic ocean to explore the wreck of the Titanic. A support ship lost contact with the sub a few hours after it began its descent. For 5 Questions On, the BBC's Science Correspondent Jonathan Amos explains the significance of the underwater noises detected by a Canadian aircraft — and why sound is the key tool to communicate underwater. Image credit: PA