"VANISHED" EXPLORES THE REAL STORY BEHIND A COUPLE'S MYSTERIOUS DISAPPEARANCE OFF THE GREAT BARRIER REEF, NOW THE SUBJECT OF THE BOX OFFICE HIT "OPEN WATER"

"Vanished: Disappearance Down Under" -- A Special Edition of "20/20" - Airs Monday, September 6 at 10 p.m., ET.

Tom and Eileen Lonergan, returning home to the U.S. from a three-year Peace Corp assignment in Fiji, decided they wanted to experience the thrill of diving off the Great Barrier Reef. So they joined 24 other divers and snorkelers on a charter boat to the reef on a perfect "flat calm" day. But, somehow, their dive boat returned to port without them. Once discovered, their disappearance made headlines worldwide. In an hour-long report that originally aired in June 2001, ABC News' Robin Roberts chronicles this remarkable story of mystery and intrigue that has now become the subject of a popular movie and, once again, has people talking. "Vanished: Disappearance Down Under" -- a special edition of "20/20" -- airs MONDAY, SEPTEMBER 6 (10:00-11:00 p.m., ET), on the ABC Television Network.

As police investigated the Lonergans' disappearance, a mystery began to unfold. Did they stage their disappearance? Were they actually still alive? Could they have committed suicide? For six months new clues surfaced. There were over 20 alleged sightings of the Lonergans in Australia. Most of their dive gear washed ashore on the same remote beach in perfect condition. Then, a plea for help written on a dive slate was discovered in a mangrove swamp nearly 100 miles away. "It was a dying declaration, it was a message in a bottle, using diving equipment. That's what it signified to me. That those people were truly left 30 nautical miles to sea and had written a plea for help, at eight o' clock the following morning," says John Bailey, a prosecutor in the coroners office. It was his choice to recommend criminal prosecution for the boat's captain. "And had the search started earlier, we may not have needed an inquiry," he adds.

Eventually, questions began to focus on the conduct of the dive boat's captain and crew. How could they have left the scene without noticing that two of their divers had not returned? "Vanished" recounts the emotional trial of the boat's captain on charges of criminal negligence, his controversial acquittal and the toll these events have taken on him and the rest of the crew.

The executive producer of "Vanished: Disappearance Down Under" is Rudy Bednar. The producer is Howie Masters.