SYDNEY (Reuters) - The southern Indian Ocean, where investigators suspect missing Malaysia Airlines Flight MH370 may have come down, is one place where a commercial airliner can crash without a ship spotting it, a radar plotting it or even a satellite picking it up.
The empty expanse of water is one of the most remote places in the world and also one of the deepest, posing potentially enormous challenges for the international search effort now refocusing on the area, one of several possible crash sites.
Even Australia, which has island territories in the Indian Ocean and sends rescue planes to pluck stricken yachtsmen from the cold, mountainous seas in the south from time to time, has no radar coverage much beyond its Indian Ocean coast.
"In most of Western Australia and almost all of the Indian Ocean, there is almost no radar coverage," an Australian civil aviation authority source said, requesting anonymity as he was not authorised to speak on the record.
"If anything is more than 100 kilometres offshore, you don't see it."
The Indian Ocean, the world's third largest, has an average depth of more than 12,000 feet, or two miles. That's deeper than the Atlantic where it took two years to find wreckage on the seabed from an Air France plane that vanished in 2009 even though floating debris quickly pointed to the crash site.
So far, search operations by navies and aircraft from more than a dozen nations have failed to find even a trace of Flight MH370, which went missing a week ago after taking off from Kuala Lumpur for Beijing and diverting from its intended flight path.
The search effort has focused mainly on the South China Sea but is now switching to the Indian Ocean after investigators, having pieced together radar and satellite tracking data, began to suspect the Boeing 777-200ER had been deliberately flown hundreds or possibly thousands of miles off course.
Searchers still face a daunting array of possible last locations for the plane, including the northern end of the Indian Ocean as well as central Asia, though investigators say it is more likely to have flown to the south than through busier airspace to the north where it would likely have been detected.
With an estimated four hours fuel left when last spotted by radar off Malaysia's northwest coast, the plane could have flown a further 2,200 miles (3,500 km) or so, assuming normal cruising speed and altitude.
Officials think, based on the available data, the aircraft flew south until it ran out of fuel and crashed into the sea, according to a source familiar with data the U.S. government is receiving from the investigation.
In the south, any debris from MH370 would have been widely dispersed by Indian Ocean currents in the week since it disappeared.
The southern Indian Ocean, between Indonesia and Australia, is broken up only by the Australian territories of Christmas Island, home to asylum seeker detention facilities, and the Cocos (Keeling) Islands some 2,000 km (1,240 miles) northwest of Perth. The Cocos Islands have a small airport to serve the islands' combined population of just 3,000 people.
Further south, the only habitation is the handful of research stations on the scattering of tiny French-run islands including Kerguelen - a group of volcanic outcrops between Africa, Australia and Antarctica. While home to several powerful astronomical scanners and radar, there is no airport and it is seen extremely unlikely the aircraft could have made it that far.
The shipping route from Western Australia north to Asia and Europe is considered relatively quiet in global shipping terms, despite the large amount of iron ore and other resources that are shipped from Australia's northwest ports.
Ships track north staying close in to the West Australian coastline and then head north through Indonesian waters into the South China Sea or northwest toward the Red Sea.
Australia's civil aviation radar extends a maximum of just 200 nautical miles (410 km) off the coast, the civil aviation authority source said, and was used only for monitoring scheduled aircraft on approach into the country and subsequent landings.
There are just two primary radars on the west Australian coast, one in Perth and one further north in Paraburdoo, which has even less range and is used to monitor mining traffic heading to the nearby Pilbara region.
Australia's Civil Aviation Authority relies on aircraft ADSB (automatic dependent surveillance broadcast) to ping information to commercial satellites, such as telecoms firm Optus' four telecommunications satellites, and back to ground control.
The source said that this was the case with flights by Emirates Airlines, which all fly over the Indian Ocean to Australia, but it did not provide a specific radar plot.
Australia does not have any government satellites.
The Australian military has an over-the-horizon radar network that allows it to observe all air and sea activity north of Australia for up to 3,000 km (1,860 miles). This encompasses all of Java, Papua New Guinea and the Solomon Islands.
While the Jindalee Operational Radar Network (JORN) extends part-way across the northern Indian Ocean, government papers online describe it as a "tripwire" in Australia's northern surveillance system, helping underpin the defense of the country from any attack originating from the north.
Local media have said its main use recently has been to track illegal immigrants approaching Australia by boat through the region's largely unguarded northern waters.
The Australian Defence Force was not available for comment on Sunday.
A potential crash site around 1,600 km (1,000 miles) northwest or west of the Australian coast would be well within the search and rescue area of the Australian Maritime Safety Authority (AMSA), one of the largest in the world.
An AMSA spokesman said no request for assistance had been received from Malaysia as of Sunday.
(Additional reporting by Morag Mackinnon in Perth and Peter Apps in London; Editing by Mark Bendeich, Lincoln Feast and Dean Yates)