Officials announce that the search for Malaysia Airlines Flight 370 will focus on a larger area of the Indian Ocean floor as the chance of finding floating debris further diminishes. (Reuters)

The sputtering search for a missing Malaysian airliner will be expanded to include a much larger swath of the Indian Ocean floor, Australia’s prime minister said Monday, signaling a daunting new phase in the bid to find the aircraft’s wreckage.

The next stage in the hunt for Malaysia Airlines Flight 370 will focus entirely on underwater exploration, forgoing the use of airplanes and vessels to spot debris on the surface. By now, Australian Prime Minister Tony Abbott said, any wreckage is likely to have “become waterlogged and sunk.”

A broader underwater search — requiring unmanned miniature submarines trawling the ocean depths at walking speed — casts doubt on the chances of ever unraveling the confounding aviation mystery. It also drastically increases the cost of the search and expands the time frame. The underwater hunt was previously focused on a small area that required two weeks to examine. The expanded area is about 22,000 square miles, or about the size of West Virginia, and could require six to eight months to fully scour.

More than seven weeks into the search, the countries involved, including the United States, have been bearing their own costs. But Abbott said Monday that Australia will seek contributions from other nations while also engaging private companies, selected with help from the Malaysian government.

“We will do everything we humanly can, everything we reasonably can, to solve this mystery,” he said at a news conference in Canberra, the Australian capital.

What happened to Flight 370?

No debris has been found from the airplane, which disappeared March 8 with 239 passengers and crew members aboard en route from Kuala Lumpur to Beijing. Its likely endpoint — the Indian Ocean off the west coast of Australia — was determined only from analysis of signals the plane transmitted to a satellite while still airborne. Based on information about the airliner’s possible coordinates and time aloft, investigators have theorized that Flight 370 ran out of fuel more than seven hours after taking off from Kuala Lumpur.

Hopes for finding the plane were bolstered earlier this month, when U.S. Navy equipment picked up deep-sea “pings” that appeared to be black-box transmissions from the aircraft. The flight-data recorder and cockpit voice recorder are equipped with batteries to power emergency beacons after a crash. On April 9, Angus Houston, chief of the multinational search, said he hoped to find the first physical evidence of the plane “in a matter of days.”

Instead, the search hit a wall. Black-box batteries have a guaranteed shelf life of 30 days, and the U.S. Navy equipment stopped picking up the signals. Then, a torpedo-like robotic submarine known as a Bluefin-21 was sent to the 2.8-mile depths. In a series of deep-dive missions, it created a sonar map of the area that search teams thought was Flight 370’s likely location. The Bluefin-21 found nothing of interest.

Abbott said officials will bring in additional equipment for the next phase of the search. But the process of organizing contracts with private companies could take several weeks. In the meantime, the Bluefin-21 will continue to scour the ocean floor.

Abbott said he was “baffled and disappointed” about the failure to find underwater wreckage based on the apparent black-box detections. He said those pings remained “the best information that we’ve got.”

Compare the depth of Malaysia Airlines Flight 370 to other milestone depths and distances.

“It may turn out to be a false lead,” Abbott said, “but, nevertheless, it’s the best lead we’ve got.”