Those aboard the submersible called Titan, the highlight of a tourist expedition that costs $250,000 per person, included British billionaire Hamish Harding and Pakistani businessman Shahzada Dawood with his son Suleman.
The wreckage of the Titanic that sank in 1912 after hitting an iceberg lies at about 12,500 feet (3,810 metres).
OceanGate Expeditions, the private company that operates the submersible, said it was "mobilizing all options" to rescue those aboard the Titan.
Reuters GraphicsBILLIONAIRE ABOARDThe expeditions to the Titanic start in St. John's, Newfoundland, before heading out about 400 miles (640 km) into the Atlantic to the wreckage site, OceanGate's website says.
Harding's stepson later wrote on Facebook that Harding had "gone missing on submarine" and asked for "thoughts and prayers".
Persons:
Hamish Harding, Shahzada Dawood, Suleman, Paul, Henri Nargeolet, John Mauger, Harding, Dawood, Joseph Ax, Kanishka Singh, Ismail Shakil, Edmund Blair, Janet Lawrence
Organizations:
Discovery, U.S . Coast Guard, Stockton Rush, Expeditions, Twitter, Reuters Graphics, Action Aviation, Facebook, Engro Corporation, SETI, Thomson
Locations:
Nova Scotia, Canada, U.S, ., Cape Cod, St, John's, Newfoundland, UAE, California, Britain, Washington, Shahid, Karachi