Twenty-five years after the Andrea Doria collided with the Stockholm and went down, divers believe they have discovered the reason: an "enormous" hole, larger than was previously believed, in the side of the ship that essentially doomed the Italian luxury liner from the moment of collision.

The discovery, made by divers on an expedition led by Peter Gimbel, puts aside previously held theories that the liner, which sank 11 hours after being struck in a foggy sea, sank so quickly because of a missing watertight door. The door, between the generating room and three tank compartments, was to have protected the generating room. The impact of the collision with the Stockholm, which resulted in the loss of 51 lives, was apparently so great the door would have made no difference.

"The hole in the generating room was so large, that it wouldn't have mattered how many doors were opened or closed," said a New York-based spokeswoman for the expedition, Lillian Pickard. "The water was coming in directly from the sea, not from another compartment."

The "Doria project" is the name of an expedition that sailed from the eastern tip of Long Island nearly a month ago in an offshore oil vessel called the Sea Level 11. The vessel has been anchored directly above the wreck of the liner, 110 miles at sea.

Gimbel is an experienced diver who was the first to explore the Doria 48 hours after she sank. His plans for the expedition were many: to discover, at last, the answer to why the ship sank -- he had been a believer in the missing door theory; to find and bring to the surface two safes, which were estimated by the optimistic to contain as much as $3 million in currency and jewels; to make a documentary of the project entitled "Andrea Doria: The Final Chapter," and, should the attempt to retrieve the safes prove successful, to open the safes on live television.

One safe has now been recovered. But with the divers exhausted and showing some health problems, and time for the expedition boat, which was leased for only a month, up today, the explorers have called an end to the expedition.

Spokesmen for the project insist, however, that Gimbel, and his wife, Elga Andersen, the film's co-producer, consider the voyage a great success.

"They feel they have accomplished what they went for," says Pickard. "There is absolutely no disappointment . . . . "

She added, however, that the decision to leave the wreck had been difficult.

"They were very close to the second safe," said Pickard, explaining that the crew wanted to end the expedition when crew members felt the time was right, not when diving conditions dictated. "The crew wanted to say when they wanted to say goodbye, not when she the ship wanted them to stay goodbye . . . they will go down for a final look to see her one more time and then they go home . . . . "

The ship, resting on its side 230 feet down in a shroud of fishermen's nets, had been, throughout this expedition, a moody mistress.

"To me she is like another woman," Andersen said, midway through the expedition, speaking of the ship and her husband's fascination with it. "Every time she gives you something, she slaps you in the face and takes something back."

The successes, for the expedition, have been, besides the apparent key to the mystery of the sinking and the safe, recovery of two sets of metal doors. Divers, working from a pressurized bell, have also recovered dozens of fine Ginori plates, their gold applique only slightly corroded after a quarter of a century beneath the sea.

But the expedition has also been beset by problems. The weather has been rough, with seas, during one squall, reaching 14 feet and winds 50 knots, and the expedition has lost days of valuable diving time. (The expedition has been estimated to cost about $1.5 million.) An underwater camera was lost, briefly. The cable that lowered the divers in their bell into the sea one day snapped, an accident that, had it occurred seconds after it did, would have sent the bell falling into the ocean, possibly killing the divers.

Diving itself has been strenuous. Using a technique called saturation diving, the divers have lived, on deck, in a small chamber pressurized to equal the force on their bodies at the bottom of the sea. ("Mother," the divers call the chamber.) At the expedition's end, it will take 48 hours for them to "decompress." Should anything happen to the expedition ship before then, the divers would be in trouble.

Divers have also, in the last week, begun to have physical problems. Gimbel, who had a serious attack of the bends in his last expedition to the Andrea Doria five years ago, has been running a fever. Some divers have been suffering respiratory problems.

These, spokesmen say, were critical in declaring the expedition over.

"They took the physical problems into consideration when they abided by the deadline," said Pickard, "they didn't want to push their luck."