More pings – presumed to be from the lost Malaysia Airlines Flight 370 – were detected overnight, raising hopes that searchers were closing in on the wreckage of the disappeared plane.
"I believe we are searching in the right area but we need to visually identify aircraft wreckage before we can confirm with certainty that this is the final resting place of MH370," said Angus Houston, head of the Australian agency co-ordinating the search.
"I'm now optimistic that we will find the aircraft, or what is left of the aircraft, in the not too distant future."
The Boeing 777, with 239 people on board, disappeared barely an hour after takeoff a month ago.
Twice, late Tuesday afternoon and then during the night, the Australian naval vessel Ocean Shield – towing a sophisticated listening device provided by the U.S. Navy – picked up signals that matched those emitted by the two flight recorders on the missing aircraft. Signals had previously been heard days earlier, but for more than 48 hours, Ocean Shield had been unable to reacquire them.
The new sounds were not in the same area as those detected on the weekend. "I'd say they are separate acoustic events," U.S. Navy Captain Mark Matthews told Reuters.
That could further complicate an already difficult search. The pings only travel a few kilometres, suggesting either one or the other of the locations may be a false lead.
Nevertheless, the new detection event raised hopes that the plane will be found.
The Ocean Shield is trawling the inky depths, 4 1/2 kilometres down with the bat-like listening device.
And with the batteries that power the "pingers" due to die soon – after a designed battery life of only 30 days – the urgency to find the wreck of Flight 370, or at least narrow the search area, is intense.
"We need to continue … for several days right up to … the point at which there's absolutely no doubt that the emergency locator batteries will have expired," Mr. Houston said.
Unless searchers can home in on the locator beacons' once-a-second pings, they will be faced with the extraordinary task of painstakingly mapping the sea bottom looking for debris and wreckage.
That could take months – or years – and cost tens, or hundreds, of millions of dollars.
For now, the listening goes on round the clock.
Unspoken, but looming over an open-ended search, is the question of who will pay. So far, nations contributing to the search – among them, China, Australia, Malaysia, the U.S., Britain and New Zealand – have paid for their own ships and planes; this is a custom in search and rescue, but not in salvage and recovery.
When Air France pilots lost control of a perfectly flyable Airbus 330, killing all 228 on board when it slammed into the Atlantic in 2009, floating debris found within days narrowed the search area. Still it took more than two years and $50-million-plus to find and recover the flight data recorders that unravelled the mystery of what happened on board Flight 447.
"Damn it, we're going to crash," one of the three Air France pilots said as the aircraft fell out of control and the pilots argued over whether they were pointed up or down.
Whether the Malaysian cockpit voice recorder will yield similar chilling details about the last minutes of Flight 370 is uncertain. First it must be found.
Far less is known about the last seven hours of Flight 370 and – unless the pings are confirmed – the search area could be far, far larger.
As the crackpot speculation that the flight was hijacked and landed on a remote air strip or blew up over a small island or was seen crashing near fishing boats off Africa has subsided, the reality remains that few hard facts are known about the fate of Flight 370.
Shortly after the last routine radioed hand-off to Malaysian air-traffic control, which should have been followed almost immediately by a call to establish contact with Vietnamese air-traffic control, someone on board the red-and-white jetliner on its overnight flight from Kuala Lumpur deliberately began to make the plane disappear.
First the transponder – the device that broadcasts the flight's identifying number, speed, altitude and heading – was switched off. Then the automatic communications package that routinely sends engine and systems performance data was turned off. Both, according to investigators, were almost certainly deliberate acts – either by a pilot, willingly or being coerced, or someone equally familiar with the complex operation of one of the world's safest jetliners.
Now invisible to air-traffic control, the Boeing 777 changed course sharply, from northeast toward China to west and back across the Malaysian peninsula. That wasn't known at the time. Only days later when military radars were scanned for unidentifiable tracks was the blip believed to be Flight 370 found headed out into the Indian Ocean.
But Flight 370 wasn't quite silent. The little communications dome atop its fuselage was still signalling an Inmarsat satellite high above the equator, just as a cellphone will occasionally "handshake" with a nearby tower whether or not any data is moving.
Detailed analysis of those hourly handshakes – there were six more of them – led searchers into the remote waters 1,500 off the Australian coast, far from any airport or air route to anywhere.
Flight 370 evidently flew for thousands of kilometres, headed roughly due south, until it ran out of fuel and crashed into the South Indian Ocean shortly after dawn on the morning its passengers – most of them Chinese citizens – were expecting to land in Beijing.
As the red-hulled Australian ship Ocean Shield trawls for fading pings, the human tragedy continues for hundreds of families.
"We'd like to be able to tell the families that we found the location, but until we can re-confirm, we should not be too optimistic," said Captain Mark Matthews, the U.S. Navy's acoustic-locater expert.
The Ocean Shield has twice detected pings matching those from emergency locator beacons like the two attached to the flight data and cockpit voice recorder on board a Boeing 777. The first series of one-a-second pings lasted for more than two hours as the ship slowly traced a set pattern. On a reciprocal track, two additional sets of "pings" – consistent with both of the beacons being detected – were heard for 13 minutes. That was more than two days ago.
On Tuesday, the so-far fruitless search for surface debris continued with more than a dozen aircraft searching a rectangle of nearly 80,000-square kilometres.