The loss of the Titanic remains the worst shipwreck in history, and an unforgettable slap at the pride of man. The great ship rests today 400 miles east of Newfoundland and 2 1/2 miles beneath the ocean surface. For decades, it remained as inaccessible as the moon; indeed, men walked on the moon years before the first TV probes found the wreck of the Titanic. Now it is possible to visit the site, using submarines that can withstand ocean pressure so strong that any leak would cut a body in two.

"Titanica," a documentary feature shot in the IMAX process, is based largely on footage filmed at the Titanic's grave. Seen on the giant, five-story, curving Omnimax screen of the Museum of Science and Industry, it is a sight to inspire awe.

I have seen earlier photography from the Titanic, rather muddy and indistinct, and so I was not quite prepared for the high quality of the photography in "Titanica." Two submersibles were outfitted with IMAX cameras, which use film stock that dwarfs conventional movies, and with newly designed lights generating 150,000 watts. The result is positively eerie: At a depth where no sunlight at all penetrates, the subs, sometimes working together, are able to light up the area so effectively that we see clearly as we drift above the bones of the great ship.

Early scenes in the film show one of the subs slowly descending, a speck of light in the darkness. The three-member crews must spend 18 to 25 hours squeezed into tiny quarters on each of their visits, and we see them playing poker and chess, and consuming giant sandwiches. At last, the ocean floor is reached, and a debate breaks out: Are those lumps of coal from the Titanic, or whale droppings? (Almost all IMAX documentaries have a regrettable tendency to alternate the sublime with the corny.) When the Titanic is finally reached, the effect is grand, sad and creepy. The documentary footage alternates with photographs taken before the ship's launching. In one extraordinary scene, the crew members use remote-controlled pincers to pick up a suitcase from the ocean floor. After some argument ("It might contain the log book!"), they replace it. Then we see a photograph of passengers standing on deck on the day of sailing. Next to one of them, on the deck, is what looks like the same suitcase.

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