The fathom 'goof'
There's a goof listed here at IMDB that a fathom is 6 feet, thus 20,000 fathoms would mean the creature came from 23 miles down, which is impossible since the deepest part of the ocean is only 7 miles deep. But, I've heard that the title doesn't necessarily mean 20,000 fathoms deep. It could be 20,000 fathoms away. Plus, this measure is never given in the movie itself, just the title. And you have to admit, the title is cool.
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Those who think they are always right are annoying to those of us who are.