tourist submersible on an expedition to the wreckage “has the potential to be a major tragedy.”“No matter what you may read in the coming hours, all that is truly known at this time is that communications with the submersible have been lost and that is unusual enough to warrant the most serious consideration,” Stephenson wrote in a Facebook post Monday.“I am most concerned about the souls aboard, whose identities have not yet been made public,” he added at the time, though the five passengers have since been identified. Stephenson and Cameron, 68, journeyed over 2 miles beneath the ocean surface to the wreckage of the Titanic in 2005.