The US Coast Guard said on Thursday that an underwater vessel has located a debris field near the Titanic in the search for a missing submersible with five people aboard, a potential breakthrough in the around-the-clock effort.The search for the missing submersible on an expedition to view the wreckage of the Titanic passed the critical 96-hour mark Thursday when breathable air could have run out, a grim moment in the intense effort to save the five people aboard.