The U.S. Navy's Bluefin-21 and its "side scan" sonar has become the focal point of the search some 2,000 km (1,200 miles) west of the Australian city of Perth, where authorities believe Malaysia Airlines flight MH370 hit the ocean after disappearing from radars on March 8 with 239 people on board. The search has centered on a city-sized area where a series of "pings" led authorities to believe the plane's black box may be located. But the Bluefin-21's searches of the largely unmapped ocean floor have been frustrated by an automatic safety mechanism which sends it to the surface when it exceeds a depth of 4.5 km (14,763 feet). On Friday, as searchers waited for the remote-control submarine to return from its fifth mission, the U.S. Navy said the Bluefin-21 had gone to a record depth of 4,695 meters (15,403 feet) in its previous mission.

PERTH, Australia (AP) — He speaks with a calm, steady voice as he tackles question after question in an attempt to explain one of the biggest mysteries the modern world has ever known: What happened to Malaysia Airlines Flight 370?