The search for a missing Titanic-bound submersible has become an investigation and salvage mission that will take an indefinite amount of time, while tributes from around the world poured in for the five people killed when the vessel imploded deep in the North Atlantic. The announcement Thursday that all aboard perished when the submersible imploded near the site of the iconic shipwreck brought a tragic end to a five-day saga that included an urgent around-the-clock search and a worldwide vigil for the missing vessel known as the Titan. The U.S. Coast Guard said the investigation into what happened was already underway.