Perth, Australia (CNN) -- 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.
'Long way to go'
A huge search is also continuing above the waves.
Friday's area of focus covers about 217,000 square kilometers (83,800 square miles) of the Indian Ocean, 1,700 kilometers (1,050 miles) northwest of the western Australian city of Perth, authorities said. A total of 14 aircraft and nine ships will scan the area over the course of the day.
Officials have repeatedly warned of a potentially prolonged hunt for the missing passenger jet, which disappeared March 8 over Southeast Asia. Long days spent combing vast tracts of ocean have so far turned up no traces.
Houston said Friday that he expects the search area to continue to be adjusted on "a semi-regular basis."
"We've still got a long way to go," he said.
In the case of Air France Flight 447, which crashed into the Atlantic Ocean in 2009, officials found debris on the surface after five days of searching. But it took them the best part of two years to locate the main pieces of wreckage, the flight recorders and many of the bodies of those on board.
With Flight 370, the search teams so far have even fewer clues to work with.
On Thursday, Australian Prime Minister Tony Abbott warned that "we cannot be certain of ultimate success in the search" for the Malaysian aircraft. He described it as the most difficult search "in human history."
Authorities are yet to provide an explanation of why the plane flew way off course or pinpoint exactly where it ended up. Investigations into the 227 passengers and 12 crew members have so far yielded no clear evidence to suggest any of them might be behind the disappearance.
The lack of hard information is frustrating for the families of those on board.
Malaysian officials held a briefing for Malaysian relatives of those aboard MH370 Thursday evening at a Kuala Lumpur hotel, but those present told CNN nothing new had emerged.
Mohammad Sahril Shaari, whose cousin Mohammad Razahan Zamani was a honeymooning groom on the plane, said the three-hour session had felt like a "waste of time."
He added, "I was hoping for some news that they had tracked the plane or some parts of it, but nothing like that happened."
Selamat Bin Omar, the father of another passenger, Malaysian civil flight engineer Mohammed Khairul Amri Selamat, said officials explained in detail the satellite data that has led investigators to the search area in the southern Indian Ocean.
But, he said, "They could not tell us if the plane crashed. They said they were still looking into it."
The Malaysian Department of Civil Aviation denied a request by Malaysian families to release the audio recording of radio communications between the pilot, co-pilot and air traffic control, two people who attended the briefing said.
The department's chief, Azharuddin Abdul Rahman, told the relatives that even the families of pilot Zaharie Ahmad Shah and co-pilot Fariq Abdul Hamid have not been allowed to listen to the recording because it is still part of an ongoing investigation, the two attendees said.
Malaysian authorities released a transcript of the recording on Tuesday.
Meanwhile, K.S. Narendran, the husband of a passenger, spoke to CNN's Erin Burnett about the baffling mystery of the plane's disappearance.
"This is an event that is so unprecedented and I think that is so significant that it can never be allowed to get off the screens, get off the radar," he said. His wife, Chandrika Sharma, was on the flight.
"My concern is that if we don't really get to the bottom of it, we cannot really be certain that we are safe and that we are secure every time we board a flight."
CNN's Elizabeth Joseph reported from Perth, Jethro Mullen reported and wrote from Hong Kong. CNN's Laura Smith-Spark, Ben Brumfield, Mitra Mobasherat and Paula Newtown, and journalist Ivy Sam contributed to this report.