By Richard C. Paddock, Lucy Craymer and Andy Pasztor 

For a month, the dozens of planes, ships and satellites scouring the Indian Ocean looking for debris from Malaysia Airlines Flight 370 have found wooden pallets, rotting fishing nets and other floating garbage. The estimated bill is tens of millions of dollars.

Ships and planes went out again Thursday, perhaps for the last time as authorities stand ready to call off the massive air and surface search. With the hunt set to go solely underwater, the inability of searchers to find any debris from the jetliner remains one of the biggest mysteries in the 40 days since it went missing.

It has left some grieving families clinging to hope that those on board may still be alive and prompted theories from aviation and ocean experts about the plane's final minutes.

John Purvis, a former chief crash investigator for Boeing Co., said a steep and fast dive into water might leave scant surface debris.

"It could blow it into so many pieces, and the pieces would be so small, that you wouldn't be able to find much debris," he said.

On the other hand, he said, a slow, gradual descent toward calm water--even with a big, widebody jet like the 777--"might give you a complete airplane [under the surface] if it was a gentle enough event," he said.

Australian officials say they are confident they have located the general area where the plane went down. But there is a wide gulf between the conclusions offered by technical experts and what some outspoken family members believe happened to the plane.

Without physical evidence, some relatives are skeptical the aircraft crashed in the ocean. They say the plane could well have landed in some isolated spot.

"I do not think it is in any sea because if it was, they would have found the broken pieces," said Mohamad Sahril Shaari, whose cousin Muhammad Razahan Zamani, 24 years old, was a passenger. "I think the plane most probably landed somewhere. You should spend time looking for the plane on land."

Like many other relatives, he demands evidence of his cousin's fate.

"Show us irrefutable proof," he said, or "we are not going to accept that they are dead."

Philip Tan, whose brother Tan Size Hiang was a flight steward on the plane, said the lack of debris gives him hope the plane landed somewhere and that his brother is still alive. "If the debris is found, that would dash our hopes of survivability," he said.

Mr. Tan said he was inclined to believe the theory circulating in some Malaysian and other media that the Boeing 777 was taken over remotely by the U.S. and flown to the distant U.S. military base on the Indian Ocean island of Diego Garcia.

This scenario has been denied repeatedly by the U.S., but still has currency in Malaysia, where conspiracy theories are relatively common and many people believe the U.S. Central Intelligence Agency had a role in the Sept. 11 attacks.

"When it comes to politics, they can deny things without blinking their eyes," Mr. Tan said. "Just like 9/11. But the truth will be revealed a year or a few years down the road."

Malaysians' trust in their government's handling of the missing plane is low, said Ibrahim Suffian, director of the Merdeka Center, an opinion research firm. A poll by the center released this week found that 54% of Malaysians feel the government is hiding information about the plane. Only 26% said they believe the government was being truthful.

In China, some family members of passengers also say they need to see something tangible to believe the plane has crashed.

"Unless there is physical evidence, we refuse to accept any conclusion from Malaysia Airlines that there are no survivors," said Che Yutian, whose 25-year-old cousin, Zhao Peng, was on the plane returning to his home in northern China after working for a year in construction in Singapore. "Right now, it's all based on guessing and deduction."

Since the search began March 17 in what has been designated as the Australian search area, 22 military and civilian aircraft have flown more than 300 sorties looking for signs of the plane. Meanwhile, 15 military and search-and-rescue ships have been patrolling the ocean's surface.

"It's an enormous area to be searching, and an area which is not well covered with surveillance assets," said British Defense minister Philip Dunne in Kuala Lumpur on Tuesday. "Had this happened in the Northern Hemisphere, I think it might have been a very different story."

Oceanographers, aviation experts and search authorities point to several possibilities for why the hunt in that area has turned up no wreckage.

For one, searchers lost valuable weeks because incorrect information led them first to the South China Sea and then to a section of the Indian Ocean south of the current search area.

During that time, strong currents and eddies in the area could have carried surface debris more than 600 miles north, east or west, said Simon Boxall, an oceanographer at the University of Southampton in the U.K. By now, he said, any remnants of the plane that could have floated likely would have sunk by now from becoming too waterlogged.

In addition, cyclone Gillian passed close by the search area in late March, said Charitha Pattiaratchi, an oceanographer at the University of Western Australia. "It would have churned up the ocean and maybe made large pieces smaller and maybe things sunk," he said.

He also pointed out that there might not have been much floating debris to start with.

"It may have impacted in such a way that very little debris was actually created," he said. "Most of the debris would be life jackets, people's belongings, water bottles and things and for some reason they could be trapped inside the fuselage."

When jetliners crash into water, the amount and size of debris can vary greatly depending variables such as the plane's altitude, the angle it went down, and its speed.

In the 1998 crash of Swissair Flight 111, the floating wreckage recovered represented only 2% of the McDonnell Douglas MD-11 widebody jetliner's total weight. The pilots lost control of the aircraft during a cabin fire and the plane crashed nose first off Canada's eastern coast at a speed of about 350 miles an hour.

After some 15 months of effort, including dredging operations, the bulk of the plane was recovered. Some of the debris was so fine that "it took a lot of reconstruction to figure out" what part of the plane it came from, recalled Nick Stoss, former head of aircraft investigations for the Transportation Safety Board of Canada.

Similarly, floating wreckage recovered from EgyptAir Flight 990, which crashed into the Atlantic Ocean in 1999 after the plane went into a nose dive, was very small in size.

In the case of the 2009 crash of Air France Flight 447 off South America, rescuers and salvage crews soon found hundreds of pieces of floating remains, including 50 bodies, uninflated life jackets, seat cushions and the plane's complete tail fin. Two years later, searchers located the jet's main wreckage on the ocean floor.

Investigators found that the Airbus A330 had stalled at 38,000 fleet due to improper commands by the pilots and quickly lost altitude. It hit the water on its belly at about 170 miles an hour and broke apart on impact.

Jeffrey Ng in Hong Kong, Celine Fernandez in Kuala Lumpur and Esther Fung in Shanghai contributed to this article.

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