An autonomous underwater vehicle hunting for missing Malaysian jet MH370 on Tuesday was set for a second sweep of the remote Indian Ocean sea bed, after aborting its first search when it encountered water deeper than its operating limits.
The US navy's unmanned vehicle loaded with sonar had earlier deployed on Monday night from the Australian ship Ocean Shield which has spearheaded the hunt for the Boeing 777 that vanished on March 8.
"After completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500 metres and its built-in safety feature returned it to the surface," a statement from Australia's Joint Agency Coordination Centre said.
"The six hours of data gathered by the autonomous underwater vehicle is currently being extracted and analysed," JACC said.
The AUV had been due to spend 16 hours collecting data.
US navy captain Mark Matthews said the vehicle had exceeded programmed operational limits and automatically resurfaced.
Search radius of 40 square kilometres
"In this case the vehicle's programmed to fly 30 metres over the floor of the ocean to get a good mapping of what's beneath," he told CNN from Perth.
Charts had put the depth at 4,200 to 4,400 metres, he said.
"It went to 4,500 metres and once it hit that max depth, it said this is deeper than I'm programmed to be, so it aborted the mission."
Matthews, a search and recovery expert, said the crew would now refine the task to cope with the depth encountered.
"It happened in the very far corner of the area it's searching. So they are just shifting the search box a little bit away from that deep water."
The Bluefin-21 would embark on a second mission during the day, weather permitting, JACC said.
The vehicle is equipped with side-scanning sonar and will initially focus on 40 square kilometres (15 square miles) of seabed in the vicinity of signals believed to be received from the plane's black box.
It is 39 days since the plane vanished and the batteries powering the black box tracker beacons had a life of only 30 days.