Deep sea challenge for MH370 search
The underwater search for wreckage from Malaysia Airlines Flight MH370 came to a sudden halt on Tuesday when the robotic mini-submarine Bluefin-21 exceeded its maximum depth and returned to the surface.
Bluefin-21 had begun its search on Monday near an area of sea bed known as the Zenith Plateau, around 1,800km north west of Perth, Australia, and close to where ultrasonic "pings" likely to have come from "black box" flight recorders were detected.
But after scanning the sea bed for six hours, a sudden depth increase triggered an automatic safety device, returning the mini-sub to the surface.
Bluefin 21 was expected to resume scanning on Tuesday, but search teams may now have to turn to other types of underwater vehicle capable of going deeper.
One such craft could be the Remus-6000, which was used in the search for the wreckage of Air France Flight 447 in 2011.
The AUV Abyss, a Remus-type owned by German Ocean Research group Geomar, could be readied at its base in Kiel and sent to Australia within two weeks.
"To our knowledge there are only two Remus type AUVs available, one is ours, and one is at Woods Hole (Oceanographic Institution) in Massachusetts, America," said Geomar spokesman Dr Andreas Villwock.
Dr Villwock explained that the Abyss mini-sub would not need any special configuration, and could be flown to Australia by air cargo and shipped to the search zone.
However, he added that no formal request had yet been received from the Australian authorities.
A spokesperson for Woods Hole said it had not received a formal request either.
The record depth for an unmanned craft is held by the Japanese mini-sub Kaiko, which reached 10,911m in the Challenger Deep section of the Mariana Trench - the deepest known part of the ocean - in 1995.
Kaiko was lost in 2003, but in 2009 the American mini-sub Nereus reached 10,902m in the same area.
The deepest manned voyage came in 1960, when the Trieste - a deep-diving submersible known as a bathyscaphe - reached 10,916m in the Mariana Trench.
In 2012, film director James Cameron became the first person to travel solo to the bottom of the Mariana Trench.
Depths in the current search zone range from 1,753m to 6,000m, but the sea bed in the region has not been extensively surveyed.
The BBC's science correspondent Jonathan Amos says that the topography of the ocean floor in the zone is unusually rugged, making the operation extremely challenging.
Current maps of the area's sea bed are mostly based on satellite surveys, and more detailed models are being assembled by the Commonwealth Scientific Industrial Research Organisation (CSIRO).
"Satellites are continuously monitoring the ocean, the principal ones for this investigation are European and US sea-level measuring satellites," said the organisation's oceanographer David Griffin.
"We're also using thermal imagery of the ocean to see the details of what the flow (current) has been."
He explained that if the location of the detected signals had correctly identified the crash site, multi-directional ocean currents could explain why no wreckage has yet been found.
"It's right at a position where the current is flowing in one direction just south of that position, and in the other direction just north of that," he said.