Question

what is one reason people used to think that nothing could live deep down in the ocean?

Answer 1

-only humans are living on earth and there's no animals
                               or
-they used to think there is no oxygen underwater

Answer 2

A reason we used to believe that nothing could live deep within the ocean is simply that we had not explored it and we were not able to explore it. Any attempts we made to have a deep sea exploration trip ended horribly.