A multinational team is desperately trying to find debris floating in the water or faint sound signals from the recorders that could lead them to the aircraft and help unravel the mystery of its fate.
Finding floating wreckage is key to narrowing the search area, as officials can then use data on currents to backtrack to where the plane hit the water, and where the flight recorders may be.
Beacons in the black boxes emit "pings" so they can be more easily found, but the batteries only last about a month. So far, there's been no sign of the Boeing 777.
Officials have said the hunt for the wreckage is among the hardest ever undertaken, and will get much harder still if the beacons fall silent before they are found.
"Where we're at right now, four weeks since this plane disappeared, we're much, much closer," said aviation expert Geoffrey Thomas, the editor-in-chief of AirlineRatings.com.
"But, frustratingly, we're still miles away from finding it. We need to find some piece of debris on the water, we need to pick up the ping."
The search will get more complicated if the signal beacons fall silent.
"What we may then do is start an enormous international effort to actually survey the Indian Ocean floor," he said. Such an effort "will take years and years to do."
The recorders could help investigators determine why Flight 370, which disappeared March 8 while en route from Kuala Lumpur to Beijing with 239 people aboard, veered so far off-course.
Two ships, the Australian navy's Ocean Shield and the British HMS Echo, carrying sophisticated equipment that can hear the recorders' pings, returned today to an area investigators hope is close to where the plane went down. They concede the area they have identified is a best guess.
Up to 13 military and civilian planes and nine other ships were also taking part in the search today, the agency coordinating the search said.
Weather conditions in the area, which have regularly hampered crews trying to spot debris, were fair with some rain expected, the Joint Agency Coordination Centre said.