The catastrophic implosion of the Titan submersible near the site of the Titanic wreckage this week may serve as a grim lesson for any company aiming to take amateur adventurers to extreme depths without heeding industry expertise, long-time undersea explorers have warned. Privately owned and operated by U.S.-based OceanGate, the safety of the Titan has been a source of concern for many in the undersea exploration community for years. Deep-sea tourism isn't a huge industry. There are a handful of companies that take paying customers deep into the ocean. But OceanGate was an outlier because of the Titan's experimental construction...