I was watching prehistoric documentaries,
and I'm just wondering if we have explored all seas on earth yet,
even if we have, did the researchers go all the way down to the bottom of the sea? or is that impossible because it is too deep and we're unable to go that far?
(How deep is the deepest sea?)

If it's No to both questions, then that's very interesting. Because there might be many many more creatures we have not found to exist yet.