by Bastien Inzaurralde, with Peter Hutchison in New York
A massive search and rescue effort for a missing submersible near the wreck of the Titanic is in a critical stage, with just hours before its oxygen supply for the five people aboard is expected to run out on Thursday.
While coast guard officials insisted they remained "hopeful," with a surge of assets and experts joining the operation and sonar picking up unidentified underwater noises, the challenge of locating and recovering the crew alive appeared increasingly formidable.
Based on the sub's capacity to hold up to 96 hours of emergency air, rescuers estimate that the passengers may run out of oxygen in the early hours of Thursday.
Organizers of the multinational response - which includes US and Canadian military planes, coast guard ships and teleguided robots - are focusing their efforts in the North Atlantic close to the underwater noises detected by sonar.
An additional Canadian vessel carrying medical staff and a decompression chamber was en route to the area early Thursday, with Canadian media reporting it was not expected to arrive before midday.
The sounds raised hopes that the passengers on the small tourist craft are still alive, though experts have not been able to confirm their source.
"We don't know what they are, to be frank with you," said US Coast Guard Captain Jamie Frederick.
"We have to remain optimistic and hopeful."
The submersible, named Titan, began its descent at 8:00 am on Sunday and had been due to resurface seven hours later, according to the US Coast Guard.
The 6.5-meter tourist craft lost communication with its mothership less than two hours into its trip to see the Titanic, which sits nearly four kilometers below the surface of the North Atlantic.
Titan was carrying British billionaire Hamish Harding and Pakistani tycoon Shahzada Dawood and his son Suleman, who also have British citizenship.
OceanGate Expeditions charges $250,000 for a seat on the sub.
Also on board is the company's CEO, Stockton Rush, and a French submarine operator Paul-Henri Nargeolet, nicknamed "Mr Titanic" for his frequent dives at the site.
Ships and planes have scoured around 20,000 square kilometers of surface water - roughly the size of the US state of Massachusetts - for the vessel, which attempted to dive about 400 miles off the coast of Newfoundland, Canada.
After the noises were detected by a Canadian P-3 aircraft, rescuers relocated two remotely operated vehicles (ROV) that search under the water and one surface vessel with sonar capability.
The ROV searches have not yielded results but data from the Canadian aircraft has been shared with US Navy experts for acoustics analysis.
"There have been multiple reports of noises and every one of those noises is being analyzed," said Carl Hartsfield, from the Woods Hole Oceanographic Institution.
He added that the sounds were "described as banging noises."
The US Coast Guard said the number of surface vessels in the search would double from five to 10 within 24 to 48 hours.
The Navy has sent a specialized winch system for lifting heavy objects from extreme depths, other equipment and personnel; and the Pentagon has deployed three C-130 aircraft and three C-17s.
Fears of a leak
Titan's mission was expected to be the only manned trip to the Titanic this year due to bad weather, Harding wrote in an Instagram post beforehand.
The Titanic hit an iceberg and sank in 1912 during its maiden voyage from England to New York with 2,224 passengers and crew on board. More than 1,500 people died.
It was found in 1985 and remains a lure for nautical experts and underwater tourists.
The pressure at that depth as measured in atmospheres is 400 times what it is at sea level.
Alistair Greig, professor of marine engineering at University College London, has suggested two possible scenarios based on images of Titan.
He said if it had an electrical or communications problem, it could have surfaced and remained floating, "waiting to be found" - bearing in mind the vessel can reportedly be unlocked from the outside only.
"Another scenario is the pressure hull was compromised - a leak," he said in a statement.
"Then the prognosis is not good."
In 2018, OceanGate Expeditions' former director of marine operations David Lochridge alleged in a lawsuit that he had been fired after raising concerns about the company's "experimental and untested design" of the craft.
Inside the sub
Tom Zaller, who runs the company behind "Titanic: The Exhibition," toured the wreck 23 years ago in a submersible much like the one that went missing Sunday.
"You're sending a very small vessel two and a half miles down, which is incredibly complicated and technical," he said.
"It's just this very seemingly unsophisticated sphere."
Zaller has known Nargeolet for decades and was in touch with Rush before he embarked on Sunday's tour.
"I was in that sub for 12 hours with everything working fine," Zaller said. "They've been there for almost four days. I just can't imagine."