In a race against the clock on the high seas, an expanding international armada of ships and aeroplanes is searching for a submersible that vanished in the North Atlantic while taking five people down to the wreck of the Titanic.

US Coast Guard officials said the search on Tuesday (Wednesday NZT) covered 10,000 square miles (26,000 square kilometres) but turned up no sign of the lost sub known as the Titan. Although rescuers planned to continue looking, time was running out because the vessel would have less than two days of oxygen left if it is still intact and functioning.

A Rolling Stone report said emails obtained by them said that Canadian aircraft had detected “banging” in 30-minute intervals in the area where the submersible disappeared.

“RCC Halifax launched a P8, Poseidon, which has underwater detection capabilities from the air,” the Department of Homeland Security emails read.

 “The PH deployed sonobuoys, which reported contact in a position close to the distress position. The P8 heard banging sounds in the area every 30 minutes. Four hours later additional sonar was deployed and banging was still heard.”

It remains uncertain what caused the banging sounds.

In another email obtained by Rolling Stone, Richard Garriot de Cayeux, president of travel and research group The Explorers Club, said the “sonar detected potential ‘tapping sounds’ at the location, implying crew may be alive and signalling’”.

De Cayeux said there was “cause for hope”.

 “We have much greater confidence that 1) There is cause for hope, based on data from the field - we understand that likely signs of life have been detected at the site,” he said in a social media statement.

Earlier reports confirmed Canadian military had dropped sonar buoys to listen for any sounds from the Titan.

Captain Jamie Frederick of the First Coast Guard District in Boston told a news conference “this is a very complex search, and the unified team is working around the clock”.

Frederick said the crew would have no more than about 41 hours of oxygen remaining as of midday Tuesday (Wednesday morning NZT). That means its air supply could run out on Thursday morning (tomorrow night NZT).

The Titan submersible is carrying five people. Photo / AP

He added that an underwater robot had started searching in the vicinity of the Titanic and that there was a push to get salvage equipment to the scene in case the sub is found.

Three C-17 transport planes from the US military have been used to move commercial submersible and support equipment from Buffalo, New York, to St John’s, Newfoundland, to aid in the search, a spokeswoman for US Air Mobility Command said.

Inside the Titan. Photo / Supplied

The Canadian military said it provided a patrol aircraft and two surface ships, including one that specialises in dive medicine.

Authorities reported the carbon-fibre vessel overdue on Sunday night, setting off the search in waters about 700km south of St John’s. At the helm was pilot Stockton Rush, the CEO of the company leading the expedition. His passengers were British adventurer Hamish Harding, two members of a Pakistani business family and a Titanic expert.

OceanGate Expeditions submersible vessel named Titan during a trip to the Titanic wreck. Photo / AP

The submersible had a four-day oxygen supply when it put to sea around 6am Sunday, according to David Concannon, an adviser to OceanGate Expeditions, which oversaw the mission.

CBS News journalist David Pogue, who travelled to the Titanic aboard the Titan last year, said the vehicle uses two communication systems: text messages that go back and forth to a surface ship and safety pings that are emitted every 15 minutes to indicate that the sub is still working.

Both of those systems stopped about an hour and 45 minutes after the Titan submerged.

“There are only two things that could mean. Either they lost all power or the ship developed a hull breach and it imploded instantly. Both of those are devastatingly hopeless,” Pogue told the Canadian CBC network on Tuesday.

The submersible had seven backup systems to return to the surface, including sandbags and lead pipes that drop off and an inflatable balloon. One system is designed to work even if everyone aboard is unconscious, Pogue said.

Eric Fusil, director of the University of Adelaide’s Shipbuilding Hub, said there are other scenarios that could cut communications, including an electrical fire that could create toxic fumes and render the crew unconscious.

Another possibility is that Titan became entangled in the wreck of the Titanic and is stuck there, Fusil said.

Adventurer Hamish Harding, one of five people on board the Titan. Photo / Action Aviation via AP

“What I would like to believe ... is that Titan suffered from a power loss, but they could still go back to the surface” and be spotted by aircraft and ships, he said.

Experts said the rescuers face steep challenges.

Alistair Greig, a professor of marine engineering at University College London, said submersibles typically have a drop weight, which is “a mass they can release in the case of an emergency to bring them up to the surface”.

“If there was a power failure and/or communication failure, this might have happened, and the submersible would then be bobbing about on the surface waiting to be found,” Greig said.

Another scenario is a leak in the pressure hull, in which case the prognosis is not good, he said.

“If it has gone down to the seabed and can’t get back up under its own power, options are very limited,” Greig said. “While the submersible might still be intact if it is beyond the continental shelf, there are very few vessels that can get that deep, and certainly not divers.”

The Canadian research icebreaker Polar Prince, which was supporting the Titan, was to continue conducting surface searches with help from a Canadian Boeing P-8 Poseidon reconnaissance aircraft, the Coast Guard said on Twitter. Two US Lockheed C-130 Hercules aircraft also conducted overflights.

OceanGate’s expeditions to the Titanic wreck site include archaeologists and marine biologists. The company also brings people who pay to come along. They take turns operating sonar equipment and performing other tasks in the submersible.

Rush told The Associated Press in June 2021 that the Titan’s technology was “very cutting edge” and was developed with the help of Nasa and aerospace manufacturers.

“This is the only submersible – crewed submersible – that’s made of carbon fibre and titanium,” Rush said, citing a design that includes 5-inch-thick carbon fibre and 3.25-inch-thick titanium.

Passengers included Harding, who lives in Dubai in the United Arab Emirates; Pakistani nationals Shahzada Dawood and his son Suleman, whose eponymous firm invests across the country; and French explorer and Titanic expert Paul-Henry Nargeolet.

Greg Stone, a longtime ocean scientist based in California and a friend of Rush, called the lost submersible “a fundamentally new submarine design” that showed great promise for future research. Unlike its predecessors, the Titan was not spherical and instead relied on a cylindrical shape that tapers at one end.

“Stockton was a risk taker. He was smart ... he had a vision. He wanted to push things forward,” Stone said.

The expedition was OceanGate’s third annual voyage to chronicle the deterioration of Titanic, which struck an iceberg and sank in 1912, killing all but about 700 of the roughly 2200 passengers and crew. The wreckage was discovered in 1985 and has been slowly succumbing to metal-eating bacteria.

OceanGate’s website described the “mission support fee” for the 2023 expedition as US$250,000 ($405,450) a person.

Recalling his own trip aboard the Titan, Pogue said the vessel got turned around looking for the Titanic.

“There’s no GPS underwater, so the surface ship is supposed to guide the sub to the shipwreck by sending text messages,” Pogue said in a segment aired on CBS Sunday Morning. “But on this dive, communications somehow broke down. The sub never found the wreck.”