Hopes that Malaysia's missing jet might soon be found are yielding to the sobering realisation of the immense challenge of searching an uncharted seafloor at depths that push deep-sea technology to its limits.

"I have compared it to sending a man to the moon. We know how to do it, but we can't just do it in three weeks," said Erik van Sebille, an oceanographer at the University of New South Wales.

Twice in two days, an advanced US Navy mini-submarine has had to abort its search of the remote Indian Ocean for wreckage from the Boeing 777, which vanished on March 8.

The unmanned Bluefin-21 bounced back to the surface on Tuesday after hitting its maximum depth of 4,500 metres (15,000 feet), and Wednesday's search was cut short due to technical trouble.

The hitches have raised the spectre of a prolonged, difficult search that may require even more sophisticated equipment to be deployed.

For nearly a month, the search effort has focused on a vast and lonely stretch of ocean where the Malaysia Airlines jet is believed to have crashed after inexplicably veering far from its Kuala Lumpur-Beijing flight path.

Underwater signals detected in the last ten days -- thought to be from the plane's "black box" -- raised hopes that wreckage could soon be found.

But these beacons, with a normal lifespan of around 30 days, have since gone silent -- forcing investigators to look below the surface in a targeted area encompassing 40 square kilometres.

In a dark, extremely deep and little-known seascape far off Western Australia, it is a daunting prospect, experts say.

- Mysterious and unmapped -

"It has not been mapped -- in fact most of the deep ocean has not been mapped," Charitha Pattiaratchi, an oceanographer at the University of Western Australia, said of the search area.

"It is very cold and dark with high pressures -- 450 times that at the surface."

Experts cannot even agree on the nature of the seascape, variously described as flat, rocky or coated in super-fine silt that could envelop and hide wreckage.

The US Navy estimates the Bluefin-21 may need "anywhere from six weeks to two months to scan the entire search area". Nothing has been detected yet.

Authorities may need to take a step back and begin seafloor mapping by ships at the surface to get an idea of the environment below, said Ian Wright, director of science and technology at Britain's National Oceanography Centre.

"It would give you an idea, for instance, of which areas were hard substrate, volcanic ridges, faults, those sorts of things," Wright said.

Afterwards, submersibles could be sent down for a closer inspection of more defined areas.

There is a gathering sense that the Bluefin-21 might not be up to the enormous task of searching a large undersea expanse at depths more than 2,000 feet lower than where the Titanic came to rest.

Angus Houston, the Australian head of the search operation, acknowledged Monday that "much larger", and deeper-diving, equipment may be needed.

"They are being looked at as we speak," he said, adding that partners in the international search will need to discuss "who has the capabilities to do this work" at such depths.

- 'Pretty extreme' -

Experts said that moving more advanced machinery into place will take time, and the mission will remain arduous -- particularly the eventual recovery of anything from the seafloor.

"There are two ways you can work at those depths. The first is manned submersibles. Very few countries have that capability," said Wright.

A "likelier option" is the use of remotely operated vehicles (ROVs) that can go as deep as six kilometres, he said.

An ROV was used to pluck the flight data recorders of Air France 447 from the bottom of the Atlantic in 2011 at depths of around 3,000 metres.

"There's only a small number of ROVs available which will operate at (the MH370 search area's) depth. It's pretty extreme," Wright said.

Air France showed it can be done but it took nearly two painstaking years even though searchers had a better idea of where it crashed.

David Mearns, a US marine scientist who led the search that in 2008 located the Australian naval vessel HMAS Sydney -- sunk in battle during World War II -- said MH370 would eventually be found.

But families, authorities, the media and a fascinated world need to be patient, he said.

"I believe the hardest part is done. They have found it," he said in reference to the beacon signals heard earlier.

"The next part is not going to be easy. It's complex and challenging. But it's now a manageable task."

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