The Disney studio's first American-made live-action spectacle remains one of its best, thanks mainly to James Mason's portrayal of Jules Verne's antihero submarine commander, Captain Nemo, as a misguided Victorian-era terrorist. Mason brings such feline assurance to the part that he makes Kirk Douglas' hound-dog overacting as captive harpoonist Ned Land a forgivable counterpoint. (But not entirely: You may want to hit ''skip'' when Douglas sings a sailor's ditty.) Disney milked every promotional angle for the film's debut, which makes for a supplemental disc overflowing with goodies in 20,000 Leagues Under the Sea Special Edition DVD. Highlights: a cut animated-fish sequence that evidently influenced Pixar's upcoming ''Finding Nemo,'' excerpts from the Disneyland TV show cannily plugging the picture, and new interview footage with Douglas and director Richard Fleischer taking affectionate swipes at each other. Asked on a commentary track whether Douglas rose to the challenge of being an ensemble player, Fleischer cracks, ''Sometimes he rose too far [and I'd] have to knock it down a bit.''