We know more about what's up there than we do about what's down there (sociopathic billionaire hubris and their fatal obsession with the Titanic be damned) and it is for that reason that it is said that only 10 or 20% of our oceans have been explored.
This means that it is extremely likely that the creatures that existed before humans on this planet, still exist at those unexplored depths, and that is the reason why I am staying firmly on this planet. I have heard that lift off and re-entry into the Earth's atmosphere are the most dangerous times and so, I am just gonna stay here where it is safe..