It seems to be miniatures of the two submarines at the beginning of the movie Starting at 1:20 you can see the real thing here: https://youtu.be/PcuKfOomVbM ht

It seems to be miniatures of the two submarines at the beginning of the movie Starting at 1:20 you can see the real thing here: https://youtu.be/PcuKfOomVbM https://youtu.be/PcuKfOomVbM I'm guessing these miniature submarine shots were overlaid on top of real shots of the titanic? And then VFX were added to make the subs look cooler. Spliced with other shots to make it look like the scientists were inside the subs. The YT comment was even more interesting than the clip