Rescuers searching for a missing tourist submersible near the wreck of the Titanic have detected “underwater noises” in the search area, the US Coast Guard said on Wednesday, with oxygen for the five on board rapidly running out more than two days after they lost contact.

All communication was lost with the 21-foot craft during its descent Sunday to see the remains of the British passenger liner, which sits more than two miles (nearly four kilometers) below the surface of the North Atlantic.

The Titan, operated by US-based OceanGate Expeditions, was built to stay underwater for 96 hours, according to its specifications — giving the five people aboard until Thursday morning before air runs out.

The US Coast Guard said on Tuesday at about 1700 GMT that it had enough air left for 41 hours, based on its specifications, which would mean a deadline of roughly 1000 GMT on Thursday. But experts say the air supply depends on a range of factors, including whether the submersible remains intact and still has power.

Engro Corp Vice Chairman Shahzada Dawood, his 19-year-old son Suleman, British billionaire Hamish Harding, French explorer Paul-Henri Nargeolet, OceanGate CEO Stockton Rush were on board the craft that went missing near the wreck of the Titanic.

Teams from the United States, Canada and France had searched more than 10,000 square miles of open sea, roughly the size of Lebanon or the US state of Massachusetts, for the submersible by Wednesday night.

“Canadian P-3 aircraft detected underwater noises in the search area. As a result, ROV (remotely operated vehicle) operations were relocated in an attempt to explore the origin of the noises,” the US Coast Guard’s First District said on its official Twitter page.

The ROV searches “have yielded negative results but continue,” the maritime military branch added.

The Coast Guard did not detail the nature or extent of the sounds that were detected, or how they were picked up.

But CNN and Rolling Stone magazine, citing internal US government communications, independently reported late on Tuesday that banging sounds were detected by Canadian aircraft at 30-minute intervals in the search area.

Rolling Stone, the first to report the news, said the sounds were detected by sonar buoys deployed in the area “close to the distress position” and that additional sonar picked up more banging four hours later.

CNN cited a US government memo also as saying that additional sounds were heard about four hours after initial banging were detected, although the news channel said the second occurrence of noise was not described as banging.

“Additional acoustic feedback was heard and will assist in vectoring surface assets and also indicating continued hope of survivors,” CNN quoted the updated government memo as saying.

It was not immediately clear if the news reports were based on the same source.

Later, the Coast Guard said ROV searches were deployed in the area where Canadian aircraft detected the undersea noises using sonar buoys.

“We don’t know the source of that noise,” US Coast Guard Rear Admiral John Mauger told CBS on Wednesday. Two ROVs and a surface vessel are being used to try to locate the source, he said.

“This is an incredibly complex site,” Mauger said, noting that metal and other objects under the water made it difficult to determine the source.

Even if the submersible is located, retrieving it presents huge logistical challenges, given the extreme conditions miles below the surface.

Rescue aid has been pouring in from around the world, with a specialised winch system for lifting heavy objects from extreme depths, other equipment and personnel due to join the rescue effort on Tuesday night, according to the US Navy.

The Pentagon said it was deploying a third C-130 aircraft and three C-17s.

Meanwhile, a French robot that can dive to 20,000 feet underwater was sent to help find the tourist submersible, its operator said.

Search for missing sub like ‘going into space’

Rescuers trying to find the orca-sized submersible are facing a gargantuan task that will test the limits of technical know-how, experts say.

“It’s pitch black down there. It’s freezing cold. The seabed is mud, and it’s undulating. You can’t see your hand in front of your face,” Titanic expert Tim Maltin told NBC News Now.

“It’s really a bit like being an astronaut going into space.”

US Coast Guard Captain Jamie Frederick told reporters on Tuesday that his organisation was co-ordinating the search.

But, he said, it was incredibly difficult, and far beyond what the coast guard would normally tackle.

“While the US Coast Guard has assumed the role of search and rescue mission coordinator, we do not have all of the necessary expertise and equipment required in a search of this nature,” he said.

“This is a complex search effort, which requires multiple agencies with subject matter expertise and specialized equipment.”

Frederick explained that rescuers were using multiple methods as they comb the vast area for the Titan, which lost contact with its mothership just two hours into its dive near the Titanic’s watery grave.

“The search efforts have focused on both surface with C-130 aircraft searching by sight and with radar, and subsurface with P3 aircraft, we’re able to drop and monitor sonar buoys.”

The effort was being augmented on Tuesday by a huge pipe-laying vessel, which has a remotely operated vehicle expected to be deployed at the Titan’s last known position.

Jules Jaffe, who was part of the team that developed the optical imaging system used to find the Titanic in 1985, said rescuers would have to look in three separate places.

“It’s either sitting on the seafloor, somewhere in the water column, or sitting on the surface,” he told ABC10 in San Diego.

“It could be in the water column. I think that’s probably the most likely place it is.”

Jamie Pringle, a professor of forensic geosciences at Keele University in Britain, said if the mini-sub had settled on the ocean floor, it could be very difficult to spot.

“The bottom of the ocean is not flat; there are lots of hills and canyons,” Pringle said, according to NBC.

Adding to the challenge: the enormous pressure four kilometers under water, around 400 times what it is on the surface.

Such pressures put enormous strains on equipment, and very few vessels can survive these depths.

Safety concerns

Questions about the safety of the Titan were raised in a 2018 lawsuit filed by OceanGate’s former director of marine operations, David Lochridge, who claimed he was fired for voicing concerns that the hull could not withstand extreme depths.

In its own court claim against Lochridge, OceanGate said he refused to accept the lead engineer’s assurances and accused him of improperly sharing confidential information. The two sides settled their court case in November 2018.

Neither the company nor Lochridge’s attorney have commented on the details of the dispute.

Months prior to the suit, a group of submersible industry leaders wrote to OceanGate warning that the “experimental” approach“ to the sub’s development could result in “minor to catastrophic” problems, the New York Times reported.

Aaron Newman, a former Titan passenger who knows some of the missing people, told NBC on Wednesday that he felt safe during his dive.

“Obviously, this is the type of exploration that’s doing things — this is not a Disney ride,” he said. “We’re going places that very few people have been.”

If the Titan managed to return to the surface, it could still be difficult to spot it in the open water, experts said. The submersible is sealed shut with bolts from the outside, making it impossible for those inside to escape without assistance.

If the Titan is on the ocean floor, a rescue effort would be even more challenging because of the massive pressures and total darkness at a depth of more than 2 miles.

Titanic expert Tim Maltin said it would be “almost impossible to effect a sub-to-sub rescue” on the seabed.