. >> logan: the deep sea between turkey and greece is a graveyard for ancient shipwrecks. many still lie undiscovered at the bottom of the ocean, fragments of history that remain beyond reach thousands of years later. now, one of the greatest undersea explorers in the world, robert ballard, is trying to uncover their secrets. you may know him as the man who discovered the "titanic," but what he couldn't say then is that he was on a clandestine mission for the navy at the time. it was a secret he kept for more than two decades until the mission was declassified. tonight, you'll hear how that mission helped him find the most famous wreck in modern history, and about ballard's many other discoveries in his 50 years at sea. and as we first reported last fall, when ballard took us with him to hunt for ancient shipwrecks off the coast of turkey, we made an extraordinary find even he didn't expect. to get to bob ballard, we had to make a journey of our own to what used to be one of the great crossroads of civilization. for thousands of years, ancient mariners passed through these