Rescuers searching for a missing submersible near the wreck of the Titanic have concentrated their efforts on a remote area of the North Atlantic where a series of undersea noises have been detected.
However, officials cautioned on Wednesday the sounds may not have originated from the vessel.
With estimates suggesting the air supply on board the submersible could run out by Thursday morning (local time), an international coalition of rescue teams has swept a vast expanse of the ocean for the Titan, which disappeared on Sunday while taking five people deep down to visit the century-old wreck as part of a tourist expedition.
The United States Coast Guard said remotely operated vehicle (ROV) searches were deployed in the area where Canadian aircraft recorded the noises using sonar buoys on Tuesday and Wednesday but have not found any sign of the Titan yet.
Coast Guard Captain Jamie Frederick said at a press conference that analysis of the noises has been "inconclusive."
"When you're in the middle of a search-and-rescue case, you always have hope. With respect to the noises specifically, we don't know what they are, to be frank with you."
Even if the submersible is located, retrieving it presents huge logistical challenges, given the extreme conditions kilometres below the surface.
Teams from the United States, Canada and France using airplanes and ships have searched more than 25,900sq km of open sea, roughly the size of Lebanon or the US state of Massachusetts.
The 6.7-metre submersible Titan, operated by US-based OceanGate Expeditions, began its descent at 8am on Sunday (local time) on Sunday. It lost contact with its parent surface vessel during what should have been a two-hour dive to the Titanic.
Sean Leet, who heads a company that jointly owns the support ship, told reporters on Wednesday that "all protocols were followed" but declined to give a detailed account of how communication ceased.
"There's still life support available on the submersible, and we'll continue to hold out hope until the very end, Leet, CEO of Miawpukek Horizon Maritime Services, told reporters in St John's in Newfoundland.
The wreck of the British ocean liner, which sank when it hit an iceberg on its maiden voyage in 1912, lies on the seabed at a depth of about 12,500 feet (3810 metres). It is about 1450km east of Cape Cod, Massachusetts, and 643 km south of St John's.
Those aboard the submersible, the highlight of a tourist adventure that costs $US250,000 ($NZ403,000) per person, included British billionaire and adventurer Hamish Harding, 58, and Pakistani-born businessman Shahzada Dawood, 48, with his 19-year-old son Suleman, who are both British citizens.
French explorer Paul-Henri Nargeolet, 77, and Stockton Rush, founder and chief executive of OceanGate Expeditions, who is piloting the craft are also on board.
A friend of Harding, Jannicke Mikkelsen, who has accompanied the British entrepreneur on other expeditions, told Reuters on Tuesday she was hoping for good news but was not optimistic. "It would be a miracle if they are recovered alive," she said.
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. 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 it 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 3.2km. Titanic expert Tim Maltin said it would be "almost impossible to effect a sub-to-sub rescue" on the seabed.
A French research ship carrying a deep-sea diving robot submersible was expected to arrive later on Wednesday (local time).
The unmanned robot is capable of diving as deep as the Titanic wreck and could help free the submersible if it is stuck, though the robot cannot lift the 9525kg Titan on its own. The robot could also help hook the sub to a ship on the surface that is able to lift it up, the operator said.
The sinking of the Titanic, which killed more than 1500 people, has long been immortalised in books and films. Popular interest was renewed by the 1997 blockbuster movie Titanic.
An unusual vessel
Some facts about the Titan, an unusual vessel.
• OceanGate Expeditions says it designed Titan to carry up to five people to a depth of 4000 metres (13,120 feet). The wreckage of the cruise ship Titanic sits on the ocean floor about 3800 metres (12,500 feet) below the surface.
• Titan weighs 9525kg (21,000 lbs), and can travel at a speed of 3 knots using electric thrusters. OceanGate says it is equipped with powerful LED lights, a sonar navigation system and high-end camera equipment. It has a single porthole.
• OceanGate says Titan has a life-support system that can keep five people alive for up to 96 hours.
• The submersible's carbon-fibre walls are 13cm thick, and the vessel is capped at each end with titanium domes, according to David Pogue, a CBS News journalist who travelled on Titan last year. He had to sign a liability waiver describing the vessel as "experimental" and acknowledging a risk of injury or death.
• Once passengers are inside, the hatch is bolted shut. Passengers generally sit on the floor, leaning their backs against a curved wall. There is a rudimentary toilet aboard.
• It is launched from a small platform that can be carried out to sea aboard a commercial ship and submerged a few dozen meters below the surface.
• The vehicle is operated using a video-game controller, media have reported. OceanGate workers on the surface ship track the location of Titan and send text messages to the pilot with navigation instructions.
