The search for missing Malaysia Airlines Flight MH370 in remote seas off Australia headed underwater today.
A US Navy high-tech black box locator has been deployed for the first time, as the battery life of the missing plane's cockpit data recorder dwindles.
Australian authorities said the so-called towed pinger locator will be pulled behind navy ship HMAS Ocean Shield.
The vessel will search a converging course on a 240km (150 miles) track with British hydrographic survey ship HMS Echo.
"The area of highest probability as to where the aircraft might have entered the water is the area where the underwater search will commence," Retired Air Chief Marshal Angus Houston, the head of the Australian agency coordinating the operation, said.
"On best advice the locator beacon will last about a month before it ceases its transmissions, so we're now getting pretty close to the time when it might expire."
On Monday it will be 30 days since the jetliner lost communications and disappeared from civilian radar less than an hour into an overnight flight from Kuala Lumpur to Beijing on 8 March.
The Boeing 777 was briefly picked up on military radar on the other side of Malaysia.
Analysis of subsequent hourly electronic "handshakes" exchanged with a satellite led investigators to conclude the plane crashed far off the west Australian coast hours later.
Sonar may help find the plane's black box voice and data recorders that are key to unlocking what happened on the flight.
Experts have warned the towed pinger locator may be of little use unless investigators can get a much better idea of exactly where the plane went into water.
They say its limited range and the slow speed at which it must be pulled behind the ship means it cannot cover large areas of ocean quickly.
Marshal Houston said the start of the underwater search in earnest did not override the need to keep searching for surface wreckage of the plane, as a find would be the most effective way to pinpoint a sub-sea hunt.
"This is a vast area, an area that's quite remote. We will continue the surface search for a good deal more time," he said.
"I think there's still a great possibility of finding something on the surface," he said.
"There's lots of things in aircraft that float. In previous searches life jackets have appeared which can be connected to the aircraft that was lost."