HrdoAcoustic Detectors
I found this from ATSB interesting:
Low frequency hydroacoustic signals present in the Indian Ocean are being examined to determine whether they can provide any information to help define the search area. These signals are recorded by hydrophones as part of the United Nations Comprehensive Nuclear-Test-Ban-Treaty Organisation (CTBTO) or the Integrated Marine Observing System (IMOS).
Way above my pay grade but wouldn't one have expected it to be done long ago?
I assume that as well as the U.N. hydrophones it is entirely possible that there are military arrays as well?