The next-to-last secret of the Andrea Doria, the sunken $30 million Italian luxury liner, is about to be divulged on nationwide television. The show is Andrea Doria: The Final Chapter (Thursday, Aug. 16, 8-10 p.m., Eastern time, check local listings for stations and times, since some stations in the West may play a delayed tape). Host George Plimpton will preside over a ceremony during which one of two safes from the vessel will be opened on camera. Until recently, this bank safe was submerged 240 feet within the sunken hulk of the vessel which went down 50 miles south of Nantucket in 1956, taking 52 people with her to the bottom.
Co-producers of the film are Peter Gimbel and Elga Andersen. Mr. Gimbel's involvement with the Andrea Doria goes way back to the day after she sank when he first dived to the wreck. The pictures he took appeared in Life magazine. In the years that followed he made many more diving expeditions to the ship. In more recent years, he has been joined by Elga, his co-producer/wife, an actress-filmmaker who also has become fascinated by the mysteries of the Andrea Doria.
In 1981, Peter and Elga made their second joint undersea expedition to the Andrea Doria (their first expedition had resulted in a documentary, ''The Mystery of the Andrea Doria,'' on CBS in 1976). Mr. Gimbel, heir to the Gimbel retailing fortune, is an accomplished filmmaker - ''Blue Water, White Death'' is his classic shark documentary.
I visited the producers in their brownstone on New York's East Side which serves as both their home and their office. The screening room, reached by a tiny private elevator, is furnished with two rows of old railroad-car seats, a grand piano covered with family pictures, and a large pull-down screen above the fireplace.
Before screening the first 90-minute segment of the documentary for me, both producers insisted that the Italian ship ''seems to become more hostile with each passing day'' in its underwater grave. The divers - among them Mr. Gimbel himself - were able to dig through tons of rubble to reach the bank safe on the port side of the ship. The purser's safe had to be abandoned, since it was apparent that time was running out.
The documentary will be airing on between 150 and 200 stations, covering almost 95 percent of all TV households in the United States through the ad hoc network organized by Telerep Inc. It will also be carried via satellite around the world. But the fact is, it is not on network television. Why?
Mr. Gimbel shrugs. ''Networks just won't take a show like this unless it is their own idea or they have a financial interest. This was neither. Anyway, perhaps the way we are doing it is the wave of the future for independent filmmakers.
Will it be possible for Gimbel and his 11 other limited partners to recoup the $2 million cost of the film?
''Not in the primary showing,'' he says. ''But there will be foreign sales and videocassettes, a condensed version, and, one hopes, repeats on, perhaps, PBS.''
With earnings so problematic, why did they do the film?
''I knew if we dived in the saturation mode (using a diving bell) we could spend eight hours on one dive; that kind of time would allow us to solve many questions. And I thought we had a good shot at the safes, too. However, when we sold participation to the partners, I said to everyone that I was not approaching them on the basis of a treasure hunt, but on the basis of working it into a good television show.''
If there is a great treasure in the bank safe - and there have been rumors of millions of dollars' worth of jewels - who will own the treasure?
''The Admiralty courts would have to make that decision. But there's usually a rule of thumb that the salvager keeps most of what is salvaged. While the ship has never been legally abandoned, the fact that the underwriters never made any attempt to salvage it in 25 years represents tacit abandonment.'' The bank safe has been immersed in seawater at the New York Aquarium at Coney Island, where it will be opened on camera. Paper experts will be present to make certain the expected cache of traveler's checks and currency does not disintegrate upon contact with the air.
Both Peter and Elga say that very strange things happened on the submerged decks of the Andrea Doria. ''Pins and shackles were undone three times. We tried not to think about it, because if you think too much you get spooked, and when you have to do a tough job you can't let yourself get spooked,'' says Elga.
''I must confess I am not superstitious,'' says her husband, ''but I do feel a certain great caution, a kind of foreknowledge that the Doria is going to hit interlopers with every rabbit punch in the book when they invade her privacy.'' In the show itself one of the divers says: ''It looks like this wreck doesn't want us to be diving on it.''
Is that why the second safe was abandoned?
''I was crazy to go for the second safe,'' says Peter, ''but we were having health problems down below and the surface crew was in bad shape. It was just common sense not to go for it. Too much of a risk.'
The first 90 minutes of ''The Final Chapter'' is the quintessential underwater adventure documentary. Without resorting to phony climaxes and transparent hokum, this quietly engrossing film manages to embrace more action than most adventure films, more pathos than most soap operas, more drama than most fictional melodramas. The relationship between Peter Gimbel and Elga Andersen is captured poignantly: Who will be able to forget the obviously terrified Elga calling ''Gimbel, Gimbel'' into the radio communications and getting no answer?
The viewer is allowed to share in all the technical problems of saturation diving - the teams lived in total isolation for 31 days inside a pressurized chamber affectionately called ''Mother.'' Practically all the major decisions as to whether to quit or continue were made on camera and, according to Peter and Elga, not one moment was faked ... or even reenacted.
''This meant missing some great moments,'' Gimbel admits sadly, ''but there were emergencies where we simply had to drop the cameras and help somebody. Well , maybe we can put it all into a fictionalized version.''
Will Peter and Elga ever go back for the second safe?
''This is the final chapter, really,'' adds Peter.''No more. ... Aside from the fact that my curiosity is completely satisifed, Elga and I want to move on. We want to do fiction films.''
Elga adds: ''If I could leave the safe closed forever, I would leave it closed, because dreams are perfect.''
But if the opening of the bank safe does not reveal the legendary jewels, it is hard to believe that Peter and Elga will not be lured back to the Andrea Doria one more time to try for the purser's safe ... the last secret.