is crucial in figuring out the fate of flight 370. that's because the cockpit recordings and flight data could provide the only explanation why the plane veered so far off course. as abc's david kerley explains, with each passing day, hope is fading. >> this u.s. navy listening device thousands of feet below the surface heard something like this. towed pinger locater heard it. it was for more than two hours. the ship, the ocean shield, it turned around and heard it again for another 13 minutes. >> two distinct pinger returns were audible. significantly, this would be consistent with transmissions from the flight data recorder and cockpit voice recorder. the ship is crisscrossing a nine square mile section of the south indian ocean listening again. searchers hope for a third hit and will send this robot down to the bottom using sonar it will map the ocean floor looking for