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Rescuers are scrambling to locate a deep-diving tourist submersible that was lost near the Titanic wreckage with five passengers on board and an estimated 40 hours of oxygen remaining. A search and rescue effort was dispatched Monday morning according to the Boston coastguard, with both US and Canadian coastguards participating in the extensive search. 

A  spokesperson confirmed to The Guardian on Sunday that “a small submarine with five persons onboard had gone missing in the vicinity of the Titanic wreck." 

According to authorities, the 21-foot vessel 'Titan' lost contact with the surface less than two hours into its descent.

OceanGate Expeditions began transporting small crews of "citizen scientists" in a five-person mini-sub for $125,000 per person two years ago. Oceangate had scheduled an eight-day, seven-night excursion to the disaster for June 12-20, according to the company's website. Six tourists had planned to leave and return to St John's, Newfoundland, Canada.

The sub explores the wreck, which is located on the ocean floor at 12,500 feet of water about 370 miles off the coast of Newfoundland.

Media reports say the three passengers were aboard British millionaire Hamish Harding, Pakistani businessman Shahzada Dawood and his son Suleman.

According to Coast Guard Captain Jamie Frederick, rescue attempts across an area of 7,600 square miles, "have not yielded any results."
 
Alluding to the sub's potential to carry up to 96 hours of oxygen, Frederick explained, "There's about 40 hours of breathable air left based on that initial report."

A Canadian P-3 plane placed sonar buoys near the Titanic ruins to listen for any potential sounds from the sub. The effort was limited to the surface of the ocean originally and expanded beneath the ocean on Tuesday. 

France's oceanographic institution announced the deployment of a deep-sea underwater robot to assist operations.

Paul-Henry Nargeolet, a professional diver and Titanic specialist, was among the crew members according to Harding.

According to unconfirmed sources, Stockton Rush, the CEO of OceanGate was also onboard. 
 
In a statement, Oceangate stated that it was "exploring and mobilizing all options to bring the crew back safely. Our entire focus is on the crewmembers in the submersible and their families."
 
Mike Reiss, an American television writer who made the trip to the wreckage last year aboard the same submarine, described the trip to the BBC as disorienting. He explained that the pressure at that depth, measured in atmospheres, is 400 times greater than the pressure at sea level.
 
Reiss recalled how "The compass immediately stopped working and was just spinning around and so we had to flail around blindly at the bottom of the ocean, knowing the Titanic was somewhere there."