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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