The fifth day of a massive search across remote North Atlantic continued Thursday as five people aboard a missing submersible near the wreck of the Titanic had just hours of air supply left.

The minivan-sized Titan, operated by U.S.-based OceanGate Expeditions, began its descent at 8 a.m. (12 a.m. GMT) Sunday but lost contact with its support ship near the end of what should have been a two-hour dive to the century-old shipwreck.

Having set off with 96 hours of air, according to the company, its oxygen would likely be depleted sometime on Thursday morning. Precisely when depends on factors such as whether the craft still has power and how calm those on board are, experts say and assume the Titan is still intact.

A remotely operated vehicle deployed from a Canadian vessel had reached the ocean floor and begun searching there for the Titan, the U.S. Coast Guard said Thursday morning on Twitter.

Rescue teams from multiple countries and relatives and friends of the Titan's five occupants took hope when the U.S. Coast Guard said on Wednesday that Canadian search planes had recorded undersea noises using sonar buoys earlier that day and on Tuesday.

But the Coast Guard said remote-controlled underwater vehicles searching where the noises were detected had not yielded results, and officials cautioned the sounds might not have originated from the Titan.

"When you're in the middle of a search-and-rescue case, you always have hope," Coast Guard Captain Jamie Frederick said Wednesday, adding analysis of the noises was inconclusive.

The French research ship Atalante, equipped with a robotic diving craft capable of reaching the depth where the Titanic wreck lies, about 3,810 meters (12,500 feet) below the surface, had arrived in the search zone as of Thursday.

The research vessel Atalante was first using an echo-sounder to accurately map the seabed in order for the robot's search to be more targeted, the French marine research institute Ifremer said.

The robot, Victor 6000, has arms that can be remotely controlled to help free a trapped craft or hook it to a ship to haul it up. The U.S. Navy is sending a special salvage system designed to lift large undersea objects.

Deep-seas adventure

The Titanic, which sank in 1912 on its maiden voyage after hitting an iceberg, killing more than 1,500 people, lies about 1,450 kilometers (900 miles) east of Cape Cod, Massachusetts, and 640 kilometers south of St. John's, Newfoundland.

The Titan was carrying its pilot and four others on a deep-sea excursion to the shipwreck, capping a tourist adventure for which OceanGate charges $250,000 per person.

The passengers included British billionaire and adventurer Hamish Harding, 58, and Pakistani-born business magnate Shahzada Dawood, 48, with his 19-year-old son Suleman, who are both British citizens.

French oceanographer and leading Titanic expert Paul-Henri Nargeolet, 77, and Stockton Rush, founder and chief executive of OceanGate, were also reported to be on board. Rush is married to a descendant of two of the Titanic victims.

"We're waiting anxiously, we hardly sleep," said Mathieu Johann, Nargeolet's editor at his publisher Harper Collins.

Sean Leet, who heads a company that jointly owns the support ship, the Polar Prince, said Wednesday all protocols were followed before the submersible lost contact.

"There's still life support available on the submersible, and we'll continue to hold out hope until the very end," said Leet, chief executive of Miawpukek Horizon Maritime Services.

Questions about Titan's safety were raised in 2018 during a symposium of submersible industry experts and in a lawsuit filed by OceanGate's former head of marine operations, which was settled later that year.

Even if the Titan were located, retrieving it would present huge logistical challenges.

If the submersible had managed to return to the surface, spotting it would be difficult in the open sea and it is bolted shut from the outside, so those inside cannot exit without help.

If Titan is on the ocean floor, a rescue would have to contend with the immense pressures and total darkness at that depth. British Titanic expert Tim Maltin said it would be "almost impossible to effect a sub-to-sub rescue" on the seabed.

It may also be difficult to find the Titan amid the wreck.

"If you've seen the Titanic debris field, there'll be a thousand different objects that size," said Jamie Pringle, a forensic geoscientist at Keele University in the United Kingdom. "It might be an endless task."