A multinational search team crisscrossed the sea and skies above the century-old wreck of the Titanic for a fifth day on Thursday, seeking a tourist submersible that went missing with five people aboard and was just hours away from the presumed end of its air supply.

The minivan-sized submersible Titan, operated by US-based OceanGate Expeditions, began its descent at 8 am (1200 GMT) on Sunday. It lost contact with its surface support ship near the end of what should have been a two-hour dive to the site of the world's most famous shipwreck, in a remote corner of the North Atlantic.

The Titan set off with 96 hours of air, according to the company, meaning its oxygen tanks would likely be depleted some time on Thursday morning. How long the air would actually last, experts said, depended on various factors, such as whether the submersible still had power and how calm those aboard remained.

Still, the countdown to oxygen depletion posed only a hypothetical deadline, assuming the missing vessel was even still intact, rather than trapped or damaged in punishing depths at or near the sea floor.

Rescue teams, and loved ones of the Titan's five occupants, took hope in US Coast Guard reports on Wednesday that Canadian search planes had recorded undersea noises using sonar buoys earlier that day and on Tuesday.

The Coast Guard said deployments of remote-controlled underwater search vehicles were redirected to the vicinity where the noises were detected, to no avail, and officials cautioned that the sounds may 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 at a press conference on Wednesday. "With respect to the noises specifically, we don't know what they are."

Frederick added that analysis of the sonar buoy data was "inconclusive."

In one highly anticipated addition to the search, the French research ship Atalante was en route late on Wednesday to deploy a robotic diving craft capable of descending to a depth well below that of even the Titanic's ruins more than 2 miles down, the Coast Guard said.

The French submersible robot, dubbed the Victor 6,000, was dispatched at the request of the US Navy, which was sending its own special salvage system designed to lift large, heavy undersea objects such as sunken aircraft or small vessels.

Drama in the deep

The drama was playing out in the icy waters beyond the east coast of Canada, where the British luxury liner RMS Titanic struck an iceberg and sank on its maiden voyage in 1912, killing more than 1,500 people.

The wreck of the cruise ship lies on the seabed at a depth of about 12,500 feet (3,810 meters), about 900 miles (1,450 km) east of Cape Cod, Massachusetts, and 400 miles 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.

Sean Leet, who heads a company that jointly owns the support ship, the Polar Prince, 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.

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

If the submersible managed to return to the surface, spotting it would be difficult in the vast open sea, and it is bolted shut from the outside, preventing anyone inside from exiting without assistance.

If Titan is on the ocean floor, a rescue would be even more challenging because of the immense pressures and total darkness at that depth. Titanic expert Tim Maltin said it would be "almost impossible to effect a sub-to-sub rescue" on the seabed.

The French submersible on its way could be used to help free the Titan if it is stuck on the seabed, although the robot cannot lift the 21,000-pound (9,525-kg) craft on its own. The robot could also help hook the sub to a surface ship capable of lifting it, the operator said.

Questions about the 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.