How does Bluefin-21 work?Bluefin-21 is a probe equipped with side-scan sonar. Such technology uses acoustic sounds to create a picture of the sea floor.
After emitting a sound the submarine then waits to receive a response, before calculating the distance from those results.
The bottom of the search area now being combed, while extremely deep - some 4,500m - it is not especially mountainous.
That said, despite the area being generally flat experts have warned that the search is a long and painstaking process.