The vessel that went missing Sunday in the North Atlantic while exploring the Titanic's wreckage is a submersible not a submarine, and there is a key difference.

The Titan, with five people on board, remained missing Tuesday even as an international search and rescue efforts were underway.

British billionaire explorer Hamish Harding, renowned French diver Paul-Henri Nargeolet and Pakistani businessman Shahzada Dawood and his 19-year-old son Suleman Dawood are on the OceanGate Expeditions’ submersible.

So what is the difference between a submarine and a submersible? The National Oceanic and Atmospheric Administration says that a submarine has enough power to leave port and come back to port under its own power.

But a submersible has more limited power and range. It needs a mother ship from which launch, to return to, and for support and communications.

The Titan's mother ship is the Polar Prince, a former Canadian Coast Guard icebreaker.

The watercraft submerged on Sunday (18 June) morning with the Polar Prince. Less than two hours later, the two vessels lost contact with each other, according to authorities.

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