NEW : Two naval vessels begin the underwater search using listening technology . They focus on the `` area of highest probability '' of where the plane may have hit water . The Navy will use a variety of techniques to locate and recover wreckage from the ocean floor, including sonar, submerging sonar equipment in water, and remotely operated vehicles (ROVs) that can be used for visual The search is `` the most difficult in human history , '' Australia 's Tony Abbott says .
NEW : Two naval vessels begin the underwater search using listening technology . They focus on the `` area of highest probability '' of where the plane may have hit water . The overall search resumes Friday with nearly two dozen ships and aircraft . The search is `` the most difficult in human history , '' Australia 's Tony Abbott says .