Rescue crews are desperately searching for the vessel that vanished in the North Atlantic Ocean on Sunday.
Search teams are racing against the clock to find a missing submersible before the oxygen supply runs out for five crew members on board.
A Canadian aircraft looking for the sub in the Atlantic Ocean has detected "banging" noises near its last known location.
It is unclear how long the banging lasted, but the discovery was shared via email with the US Department of Homeland Security.
The US Coast Guard, which is leading the search, has relocated its rescue operation as a result of the underwater sounds, but so far has "yielded negative results".
Authorities estimate the vessel has around 30 hours of oxygen left, as of Wednesday morning.
Canadian authorities reported the carbon-fibre vessel missing late on Sunday night, sparking an international rescue effort in waters about 700 kilometres off the coast of Newfoundland, Canada.
The sub was on an expedition to visit the wreckage of the Titanic, descending some 3,800m to the seabed.
Aboard were a pilot, a renowned British adventurer, two members of a Pakistani business family and a Titanic expert.
Named the Titan, the sub had a 96-hour oxygen supply when it set off at roughly 6 a.m. on Sunday, according to David Concannon, an adviser to OceanGate Expeditions, which oversaw the mission.
That means the oxygen supply could run out as early as 6 a.m. Thursday morning.
Journalist David Pogue, who went on an expedition to the Titanic aboard the Titan last year, said the vessel 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 vessel was 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 said on Tuesday.
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 using buoyancy.”
“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,” he 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.”
Even if they could go that deep, he doubts rescuers could attach to the submersible.
By Tuesday morning, an area totalling 10,000 square miles had been searched, the US Coast Guard tweeted.
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.
The Canadian military dropped sonar buoys to listen for any possible sounds from the Titan.
OceanGate’s expeditions to the Titanic wreck site include archaeologists and marine biologists. The company also brings people who pay €230,000 to come along, known as “mission specialists.”
They take turns operating sonar equipment and performing other tasks in the submersible.
The Coast Guard said Monday that the Titan carried a pilot and four “mission specialists." However, OceanGate's website suggests that the fifth person may be a so-called “content expert” who guides the paying customers.
The Titanic wreck lies 3,800 metres down at the bottom of the Atlantic, 400 nautical miles off the coast of Canada.
It sunk in April 1912 after striking an iceberg on its maiden voyage. Of the 2,200 people onboard, just 706 survived.
The wreck was discovered in 1985. A large amount of debris surrounds the destroyed ship.
In recent years, small submersibles have started taking tourists to visit.
OceanGate's first dive happened in 2021, says the company on its website.
Each of its expeditions is supposed to have a scientific objective, with each dive taking eight hours from top to bottom.
