A robotic submarine deployed to search for the missing Malaysia Airlines plane in the southern Indian Ocean has had its first mission cut short.
The Bluefin-21 was sent to search the sea floor for wreckage after signals believed to be consistent with "black box" flight recorders were detected.
But the drone exceeded its operating limit of 4,500m (15,000ft) and was brought back to the surface.
It is due to be relaunched in the next few hours if weather conditions permit.
Flight MH370 went missing on 8 March with 239 people on board. It was flying from Kuala Lumpur to Beijing when it lost contact with air traffic controllers over the South China Sea.
Malaysian officials believe, based on satellite data, that it ended its flight thousands of kilometres off course, in seas west of the Australian city of Perth.
Amid a major international search, an Australian navy vessel last week detected four acoustic signals that officials believe could come from the missing plane's flight recorders.
No signals have been detected since 8 April, however, amid fears that the recorders' batteries - which last about a month - have run out.
On Monday, Air Chief Marshal Angus Houston, who heads the agency co-ordinating the search for the missing plane, said it was time to deploy the Bluefin-21 to begin its search of the sea floor. It set off on its first mission on Monday night.
"After completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500m and its built in safety feature returned it to the surface," the Joint Agency Co-ordination Centre said in a statement.
"The six hours of data gathered by the Autonomous Underwater Vehicle is currently being extracted and analysed."
"Bluefin-21 is planned to redeploy later today when weather conditions permit."
Bluefin-21 is an almost 5m-long vehicle that can create a sonar map of the sea floor. On Monday officials said each mission was expected to last 24 hours, with 16 hours spent on the ocean floor, four hours' diving and resurfacing time, and four hours to download data.
But the submersible has a safety feature that brings it to the surface if it exceeds its performance capabilities.
Australian officials have said previously that they are confident they are searching in the right area for the missing plane.
But ACM Houston warned on Monday that the search of the sea floor could be a long, painstaking process that might not yield results.
Officials have no idea yet why the plane diverted so far from its intended flight path. Investigators are looking at options including hijacking, mechanical failure, sabotage and pilot action.