The hunt for Malaysia Airlines Flight 370 has penetrated beneath the waves as searchers race to catch pings from the missing plane's flight data recorders before they fall silent.
But the area of the southern Indian Ocean where British and Australian naval ships are deploying sophisticated listening technology remains nothing more than an educated guess at where the plane may have hit the water.
The British Royal Navy survey ship HMS Echo and the Australian naval supply ship Ocean Shield began searching the ocean's depths along a single 240-kilometer (150-mile) track Friday, said retired Air Chief Marshal Angus Houston, the head of the Australian agency coordinating the search efforts.
The Ocean Shield is equipped with high-tech gear borrowed from the United States: the TPL-25, a giant underwater microphone that will listen for the pings from the flight data recorders, and the Bluefin-21, an underwater robot that can scour the ocean bed for signs of wreckage. The HMS Echo also has advanced sensor equipment.
Time is running out in the efforts to detect the pings as the batteries that power the recorders' beacons are expected to expire in the coming days.
"If they do find it, I think it'll be remarkable," said Bill Schofield, an Australian scientist who worked on developing flight data recorders.
Nearly four weeks have passed since the jetliner vanished with 239 people on board. With investigators still apparently stumped by the case, information in the flight recorders could help them unravel the mystery of what happened the night the plane dropped off radar.
But there are no new clues behind the area where the underwater search is concentrated. It's based on the same kind of analysis of radar, satellite and other data that investigators have used to determine a series of shifting search areas in recent weeks.
"The area of highest probability as to where the aircraft might have entered the water is the area where the underwater search will commence," Houston said at a news conference Friday. "It's on the basis of data that arrived only recently, and it's the best data that is available."
'Just a guess'
Until searchers can find a confirmed piece of debris from the plane, which would give them a clearer idea of where the main bits of wreckage might be located, there is no certainty the technology is being pointed in the right direction.
"Really the best we can do right now is put these assets in the best location - the best guess we have - and kind of let them go," U.S. Navy Cmdr. William Marks told CNN. "Until we get conclusive evidence of debris, it is just a guess."
Searching with the pinger locator trailing from a ship is painstaking work, another U.S. Navy official said.
"It is a very slow proceeding search, 2 to 3 knots depending on the depth," said Capt. Mark M. Matthews, director of ocean engineering. But since it doesn't rely on daylight, the device can keep searching 24/7.
"It's going to take time," Matthews said, adding that the Bluefin-21 robot would only be deployed if the searchers get a clear fix on the beacons sending out the pings.
The ocean in the general area where the search is taking place is between 2,000 meters and 4,000 meters (6,500 feet and 13,000 feet) deep. The pinger locator can search as deep as 20,000 feet (6,100 meters), according to the U.S. Navy.
See updates on this story at CNN.com.