What a great movie that is! Who can forget that rat breathing liquid oxygen (or whatever they called it), and the water creature making her face to say hello. It seems sort of forgotten now (perhaps it's just me?) compared to other good films from the same era.
I remember seeing the movie as a child with my parents and I was quite appaled by that scene (I hate seeing animal abuse). My father ensured me that was just some kind of effect trickery (I think he believed so himself). Later I found out that it was indeed real.
The liquid is called fluorocarbon.
https://en.wikipedia.org/wiki/Liquid_breathing#Films_and_tel...
Wow, we do learn something new everyday :-)
The effects were so good for their time as well, a semi-forgotten gem that film.