Intensive search efforts are under way to try and find flight MH370, which vanished from radar en route to Beijing from Kuala Lumpur almost three days ago, with 239 people on board.
None of the debris and oil slicks spotted in the water have so far been linked to the disappearance - but how could a plane vanish without a trace?
Australia has vowed the search for the missing Malaysia Airlines plane will go on indefinitely, despite no sightings yet of wreckage in the Indian Ocean.
Deputy PM Warren Truss said the operation would go on until "further searching would be futile - and that day is not in sight".
Satellites detected debris this week, but so far it has not been found.
Flight MH370 dropped out of contact an hour after leaving Kuala Lumpur for Beijing on 8 March carrying 239 people.
Malaysian officials suspect the plane was deliberately taken off course.
Britain's Daily Telegraph published a transcript of communications between the pilots and Malaysian air control, although it appeared to throw little light on the reasons for the disappearance.
'Very remote'
Satellites detected possible debris earlier this week 2,500 km (1,550 miles) south-west of Perth in Australia.
The search in the Indian Ocean is being led by the Australian Maritime Safety Authority (Amsa).
It despatched six planes to the area on Saturday, to search an...MORE ON THE NEXT PAGEarea roughly the size of Denmark. Additional vessels supplied by China, Japan and the United Kingdom are due to join them in the search.
The first plane has now returned, with no success in finding debris.
Mr Truss, who is the acting PM while Tony Abbott is in Papua New Guinea, said: "It is a very remote area, but we intend to continue the search.
"At this stage we are planning for it to continue indefinitely, although I recognise that there will be a time at some stage if nothing is discovered where a further re-appraisal will have to be made but we are not even thinking about that at the present time."
Mr Abbott said from Papua New Guinea: "I understand that conditions in the southern Indian Ocean are better today, so we've got six aircraft in the area. Later today, my understanding is that an Australian naval vessel (HMAS Success) will be in the area.
"There are aircraft and vessels from other nations that are joining this particular search, because tenuous though it inevitably is, this is nevertheless the first credible evidence of anything that has happened to flight MH370."
Royal Australian Air Force group captain Craig Heap said there was a ''reasonable'' chance of the search finding something.
Each aircraft is able to search for no more than two hours because of the distance from land.
Bad weather on Thursday hampered the search using radar so on Friday the planes flew below cloud cover for a "visual search".
John Young of Amsa said planes were "flying relatively low" with "very highly skilled and trained observers looking out of the aircraft windows... to see objects".
Two merchant ships are currently involved in the search
China - which had 153 of its citizens on board flight MH370 - is sending three navy vessels as well as its icebreaker Xue Long (Snow Dragon).
'Long haul'
The search effort in the southern Indian Ocean is only part of a much wider hunt for the plane - reaching as far north as Kazakhstan.
On Saturday, India informed Malaysia that analysis of its radar records had revealed no evidence of flight MH370 crossing Indian airspace.
Satellite data, picked up some seven hours after the plane lost radio contact, suggests it could have disappeared in two corridors to the north and south of its last known location in the Malacca Straits.
Malaysia's Acting Transport Minister Hishammuddin Hussein thanked the 20 or so countries involved in the search and said they were facing a "long haul".
