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The search for missing Malaysia Airlines Flight 370 is shifting entirely to the Indian Ocean floor and away from possible surface debris, Australian officials announced Monday.

The surface search is being curtailed because debris from the Boeing 777-200ER that disappeared March 8 with 239 people aboard should have become waterlogged and sunk by now, according to Australian Prime Minister Tony Abbott.

Despite the search projected to cost $60 million, Abbott said the underwater search would be expanded to cover more of the ocean floor. An Orion 3C plane will still be on standby in case wreckage is seen, he said.

"This is probably the most difficult search in human history," said Abbott. He said the search "is entering a new phase."

No debris from the plane has been confirmed nearly two months after it disappeared on a flight from Kuala Lumpur to Beijing. Ships were unable to confirm initial satellite photographs and plane sightings of possible wreckage from the flight.

Searching the ocean surface presented challenges from the start because planes were operating at the limits of how far and how long they could fly to scan an area more than 1,000 miles from Australia.

So far, 10 civil aircraft and 19 military planes have flown during a total 41 days out of 52 since the plane went missing, Abbott said. The planes covered more than 1.7 million miles during 334 flights.

Even Monday, nine planes and 12 ships were searching, according to the search's Joint Agency Coordination Center. But Abbott said the search would shift underwater, after contracts are awarded during the next several weeks for sonar equipment that can be towed behind ships, to join the Bluefin-21 submarine already in use.

"Enormous efforts will continue to be made," Abbott said.

Surface ships detected pings presumed from the plane's voice and data recorders until April 8. But the underwater search wasn't able to find the recorders based on those signals, which Abbott called baffling and disturbing.

The autonomous Bluefin-21, which is leased by the U.S. Navy, is in its 16th day of searching the ocean floor with sonar. Technicians program the sub and then retrieve it to download data and look for echoes from man-made objects rather than rocks on the ocean floor.

But the Bluefin-21 moves at about walking speed. So far it has searched 155 square miles and Abbott said the primary underwater search zone is 21,622 square miles, which is expected to take months longer even with additional search equipment.

"If everything goes perfectly, I would say we'll be doing well if we do it in eight months," said Angus Houston, a retired air chief marshal who is leading the search for Australia and who warned that weather and equipment failures could slow things down.

One difficulty of towing sonar devices behind a ship is that the ocean is nearly 3 miles deep in this area, with the line running from the ship much longer. Dave Gallo, the project leader at Woods Hole Oceanographic Institution that found Air France Flight 447 at the bottom of the Atlantic, said ships have trouble turning quickly or tightly while dragging such a long cord in a search pattern often described as mowing the lawn.

"Imagine trying to mow the lawn by towing a lawn mower behind a helicopter," Gallo said. "It's not the best way to do things."

The $60 million cost estimate would make the search one of the costliest in history. Search and rescue operations for Air France Flight 447, which took nearly two years after the 2009 crash, cost a total of 80 million euros from France, Brazil and the USA, according to the crash report.

"We will be seeking some appropriate contribution from other nations involved, but we will ensure that this search goes ahead," Abbott said.

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