Searchers are about to send an "autonomous underwater vehicle" four kilometres underwater to look for the wreckage of flight MH370 on the sea floor.
The latest stage of the search for the missing Malaysia Airlines plane will begin within hours, former Australian Defence Force chief Angus Houston said in Perth.
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MH370: Yellow submarine to search for debris
The Bluefin-21 is the latest hope in the search for missing Malaysia Airlines flight MH370. But what is it?
"The deployment of the autonomous underwater vehicle has the potential to take us a further step towards visual identification, since it offers a possible opportunity to detect debris from the aircraft on the ocean floor."
There have been no signs of the missing aircraft since four "ping" signals were detected in an area off West Australia last Tuesday, in what was described at the time as the most promising lead so far.
Air Chief Marshal (ret) Houston, who heads the multi-agency and international search effort, said the Ocean Shield vessel will stop searching for signals using the "pinger locator". The autonomous underwater vehicle Bluefin-21 will take over.
The Bluefin-21 will searching a targeted area measuring five kilometres by eight kilometres, deemed to be the most likely resting spot for MH370, which disappeared off flight radars 38 days ago.
However, Mr Houston was cautious about the possibility of success, saying the more localised search may return nothing.
"I would caution you against raising hopes that the deployment of the autonomous underwater vehicle will result in the detection of the aircraft wreckage. It may not," he said in Perth on Monday morning.
"However this is the best lead we have and it must be pursued vigorously. Again I emphasise that this will be a slow and painstaking process."
Each trip taken by the Bluefin-21 will take 24 hours. If the first search finds nothing, the search area will be expanded.
It takes two hours for the Bluefin-21 to reach the sea floor, Mr Houston said. At walking pace, it will then search for 16 hours, before taking another two hours to return to the surface. It then takes four hours to download the data the vehicle has detected.
The vehicle is equipped with side-scan sonar which transmits an active pulse. This produces a high-resolution three-dimensional map of the sea floor.
Meanwhile, ADV Ocean Shield detected an ‘‘oil slick’’ in the range of its search zone on Sunday, Mr Houston said.
However, it will be days before the origins of the oil can be investigated by experts on shore.
As many as 11 military aircraft, one civilian jet and 15 ships were searching an area about 47,644 square kilometres for the missing flight on Monday.
There have been no acoustic detections picked up in the underwater search area since Tuesday, JACC has confirmed.
The centre of the search zone is 2200 kilometres north-west of Perth, Western Australia, which has become the headquarters for the recovery operation.
Although the search is now in its fifth week, not a single piece of physical evidence has been located to confirm the whereabouts of MH370.
The Bluefin-21 Artemis autonomous underwater vehicle can operate at a depth of 4.5 kilometres and travels at a maximum speed of 4.5 knots, or less than nine kilometres an hour.
Prime Minister Tony Abbott gave the last indication of the underwater search zone, of about 2000 square kilometres, when speaking to Chinese media on Saturday.
Under optimal conditions the submersible can complete a 25-hour mission if it maintains a speed of three knots, according to its manufacturer, Bluefin Robotics.
It has been more than a week since the batteries from the flight’s two black box beacons was due to expire and six days since signals consistent with aircraft black box ‘‘pings’’ were last detected.
On Saturday in Beijing Mr Abbott said the search was still focused on an area measuring 40 kilometres by 50 kilometres in size.
He said search authorities had hoped to reduce the area to within one kilometre before deploying the Bluefin-21.
On Monday, Mr Houston stopped short of saying he believed the batteries were dead, but he has previously said the submersible would not be dropped to the ocean floor until there was absolutely no hope the black boxes would emit another signal.
Malaysian Airlines flight MH370, with 239 people on board, vanished after mysteriously changing course during a flight from Beijing to Kuala Lumpur on March 8.
It is hoped the plane’s two black boxes: a flight data recorder, and a cockpit voice recorder will hold vital information about what happened in those final hours.
Earlier today Malaysian authorities revealed they were uncertain about what to do with the black boxes from Flight 370 if they are located and brought to the surface.
The country’s Attorney-General, Abdul Ghani, has flown to London to consult with the United Nations Civil Aviation Organisation and other experts about who should get custody of the boxes.
Although Malaysia is the head of the investigation under international law, the government has called on international experts from countries including the United States and Britain to assist with the investigation.
As the suspected final resting place of MH370 falls within Australia’s search and rescue zone, Australia has been the lead agency for coordinating the search Indian Ocean search effort
with Lindsay Murdoch