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The search continued Tuesday morning for a submersible that went missing in the North Atlantic on its way to the wreck of the RMS Titanic with rescuers speculating on how much oxygen is left for the five people onboard the vessel.

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U.S. and Canadian military resources have been searching for the craft since a research ship that launched the submersible reported it had lost contact with the vessel on Sunday, according to the BBC.

The group was diving to the Titanic wreck site about 900 miles east of Cape Cod, the U.S. Coast Guard said on Monday.

While the search continued overnight, rescuers voiced concern over a dwindling emergency oxygen supply available to the people in the submersible. As of 5 p.m. Monday, U.S. Coast Guard Rear Admiral John Mauger estimated the sub had between 70 and 96 hours of emergency oxygen left available to the five on board.

Here’s what we know Tuesday morning about the submersible and the people onboard:

What happened to the vessel?

Contact with the submersible, called Titan, was lost about one hour and 45 minutes into the dive, according to the U.S. Coast Guard. The vessel was reported overdue at 9.13 p.m. local time on Sunday by the research vessel Polar Prince.

OceanGate, a U.S. company, runs the expedition that takes customers to see the resting place of the RMS Titanic which sunk in 1912 after hitting an iceberg on its maiden voyage. The Titanic expedition is part of an eight-day trip that costs $250,000 per person.

What is the Titan?

The Titan is a 22-foot-long submersible that can seat five people. It can dive to about 4,000 meters or close to 2 ½ miles, according to OceanGate.

The vessel has emergency oxygen supplies that last up to 96 hours, according to the company.

Who is believed to be on the submersible?

While the names of the people onboard the submersible have not been officially released, several media outlets are reporting that family and friends have said via social media that these are the people who are on the Titan:

· Stockton Rush, 73, the CEO and founder of OceanGate Expeditions

· Frenchman explorer and Titanic expert Paul-Henry Nargeolet, 73,

· Pakistani businessman Shahzada Dawood, 48, and his son, Suleman, 19

· British businessman and explorer Hamish Harding, 58

What was the Titan doing when contact was broken?

The vessel was headed to the site of the wreck of the Titanic, an ocean liner that hit an iceberg in the North Atlantic on its maiden voyage in 1912. The ship sunk in about two hours, and its wreck sits some 13,000 feet below the ocean’s surface about 370 miles off the coast of Newfoundland, Canada

What is the difference between a submersible and a submarine?

According to the National Oceanic and Atmospheric Administration, the difference between a submarine and a submersible is a submersible requires a “mother ship” that can launch and recover it. The Titan’s support ship was the Polar Prince, a former Canadian Coast Guard icebreaking ship, according to the ship’s co-owner Horizon Maritime.

Also, a submersible cannot replenish its air supply as a submarine can.

What could have happened?

It’s unclear what could have happened with the vessel. Speculation ranges from a failure of systems on the submersible to the possibility it has been snagged on something.

How hard will it be to find the Titan?

Frank Owen, former director of the Australian submarine escape and rescue project told Radio 5 Live that the submersible won’t have distress beacons they can release to the surface.

“There will be radio transmitters, GPS signals,” he says. “There’ll be strobe lights and radar reflectors to help the searching forces find them.”

However, Owen said, the distress signals can’t be sent until the craft has reached the surface. To get to the surface, the vessel has “several hundred kilos of metal” that can be dropped in order to make the submersible more buoyant and send it to the surface.

“That doesn’t require power to do,” he adds. “You can have a hydraulic hand pump to cut the cable and you would have expected that to have occurred.”