also the only hard evidence available to investigators and malaysia's prime minister at the time, najib razakheir answer to me was, we are as sure as we can possibly be. >> he needed to be sure because based on those calculations, the prime minister was about to deliver some very somber news. >> flight mh370 ended in the southern indian ocean. >> the southern indian ocean, thousands of miles away, where no one could likely have survived. family members were shocked, distraught, and angry. there would be no rescue. one last hope remained. could they find the black boxes before they stopped emitting pings? >> you're not in an ivory tower. you haven't got the luxury of time. you've got pingers that may expire, so you've got to say, this is our best guess now. >> their best guess is a remote area more than twice the size of california. >> good morning. these are all the aircraft flying today. >> the australians take over the search. and soon after the australian ship "ocean shield" lowers its pinger locater into the water, pings are detected. >> clearly this is a most promising lead. >> it was wo