Titan sub: How do surveillance planes spot ocean submersibles?

Ever since the Titan submersible was confirmed lost in the Atlantic this week, planes have been combing the ocean to hunt for it beneath the waves.

On Wednesday, the US coastguard announced that a Canadian P-3 aircraft had identified unexplained underwater noises, apparently banging at half-hour intervals. The signal continues to be investigated and analysed, say officials.

How can aircraft flying high above the waves detect something so deep underwater?

Hunting for submersibles is traditionally the exclusive job of some of the largest and most technologically-advanced aircraft in any air force. Often based on civilian designs, these machines deploy a suite of impressive-sounding sensors to locate military submarines under the sea. It's usually a cat-and-mouse game between aircraft and submarines that want to stay hidden. That's not the case here.

The fact that these aerial hunters are packed with advanced new technology would seem to give them the advantage. Yet as the lost Titan sub shows, submersibles remain very difficult to find, especially at depths of 3.8km (12,400ft) where the wreck of the Titanic is found.