The authorities are cancelling air and ship searches for the missing Malaysia Airlines plane and have switched to a larger underwater operation that will cost A$60 million (S$69.9 million) and could take at least eight months to complete.
Australian Prime Minister Tony Abbott said it was "highly unlikely" that floating debris would be found, and the operation would now focus on an underwater search across an area of the Indian Ocean spanning 700km by 80km.
Searchers will continue to use an unmanned Bluefin-21 submarine operated by the United States Navy, but will also look to hire private contractors to deploy towed side-scan sonar equipment to scan the seabed for wreckage.
Unlike the Bluefin-21, which emerges after each mission to have its data downloaded, the side-scan equipment can scan and produce imagery as it is towed.
An Australian aircraft will remain on standby in case wreckage is identified, while a team of ships from Australia, Malaysia and China will continue searches until the new arrangements are put in place over the coming weeks.
Showing little of the enthusiasm of three weeks ago - when he said the plane's location had been narrowed to "within some kilometres" - Mr Abbott admitted that the authorities were "baffled and disappointed" by the failure to find wreckage. They will now consult aviation experts from Malaysia, China, Australia, Britain and the United States, who are in Kuala Lumpur, on the new search zone.
"It is highly unlikely, at this stage, that we will find any aircraft debris on the ocean surface," he said.
"Fifty-two days into the search, most material would have become waterlogged and sunk… What we are going to do, though, is enter a new phase of search focusing under the sea."
The new phase, he said, would cost A$60 million. Australia will pay, but seek contributions from "other nations involved".
"No one should underestimate the degree of responsibility that Australia has here because this has happened in our search and rescue zone," he said.
Search coordinator Angus Houston said the new phase would take at least eight months.
The side-scan equipment will be towed along a 700km arc which the plane is believed to have followed; the arc matches both a series of satellite pings emitted by the aircraft and a flight path from Cocos Islands to Perth. The water in the area has a depth of about 4,000m to 4,500m.
An initial underwater search had focused on an area where the authorities believe they detected signals from the plane's black box locator beacon.
Four sets of signals were heard before the beacon's batteries are believed to have died. The signals are being used to help determine the ongoing search area.
"It may turn out to be a false lead but, nevertheless, it is the best lead we have got, and we are determined to pursue it," Mr Abbott said yesterday of the signals.
The Boeing 777-200ER, carrying 239 people, disappeared on March 8 during a flight from Kuala Lumpur to Beijing.
In the 41 days since an international air and sea search began focusing on the Indian Ocean off the coast of Western Australia, more than 4.5 million sq km has been searched. Some 334 search flights have been conducted involving 10 civilian and 19 military aircraft from Australia, New Zealand, the US, China, Japan, South Korea and Malaysia. Fourteen ships from Australia, China and Britain have also been involved.
Mr Abbott said: "Enormous efforts have been made. Enormous efforts will continue to be made. This is an extraordinary mystery. We will do everything we reasonably can to resolve it."