Breathable air in the missing Titan submersible is expected to have run out as it nears the critical 96-hour mark since it was launched in the North Atlantic on Sunday morning, according to BNO News. 

The search mission for the missing vessel, with five people aboard, continues.

The minivan-sized Titan, operated by U.S.-based OceanGate Expeditions, began its descent at 8 a.m. (1200 GMT) on 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 some time on Thursday morning. Precisely when this is depends on factors such as whether the craft still has power and how calm those on board are, experts say, and assumes 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 on 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.

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 below the surface, had arrived in the search zone as of Thursday.

The Titan was carrying its pilot and four others on a deep-sea excursion to the the wreck of the Titanic, which sank in 1912 on its maiden voyage, 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.

(With input from Reuters)