Submarine Used for Tours of Titanic Wreckage Goes Missing in Atlantic Ocean

A submarine that gives underwater tours of the sunken Titanic has gone missing during a recent expedition ... and now, authorities are on the hunt to find it somewhere in the Atlantic.

The deep-sea vessel vanished sometime over the weekend, with a full-blown search and rescue operation launched Monday, via the U.S. Coast Guard. Reports say inside the sub were 4 crew members and one tourist.

Unclear where exactly it lost touch with its home base ... but reports say the submarine had begun a 12,500-foot descent into the ocean to see the Titanic wreckage up close off the coast of Newfoundland, Canada. A famed (and rich) British world explorer, Hamish Harding, is said to be the lone civilian accompanying the crew ... as confirmed by his family.

Harding -- who once paid top-dollar for a ride in Jeff Bezos' Blue Origin rocket -- posted a proud pic of himself Sunday, hyping up the fact that he'd be the last person to see the Titanic in '23. The org that owns and operates this submarine, OceanGate Expeditions, was actually set to resume voyages in 2024 -- and this one was going to be their last of the year.

OceanGate has said this sub belongs to them ... and that their team had yet to return from the mission. They're a private company that offers tours of the Titanic ... and tickets for this go as high as $250k.

Normally, these OceanGate Titanic trips last 8 days total and the dives to the Titanic can reach up to 10 hours apiece. OceanGate says they often rely on satellite tech to stay in touch with their subs -- but in this case, it seems all communication has been lost.