While most people see an ocean, and don't really think twice about it, the reality is we've only explored a relatively small portion of our total seafloor. Experts estimate that only 5-10% of the seafloor has been explored, and this might have something to do with the fact of just how deep it is.
Our oceans are incredibly deep, and at some points, they're deeper than the height of the highest point on Earth: Mt. Everest. In other words, if you took Mt. Everest and you put it in the deepest part of the ocean, you wouldn't be able to see it because it'd be submerged.
While mankind has tried over and over again to explore the deepest parts of the ocean, the pressure becomes so great that humans can't get down there; it's so strong that our skulls would imply implode. Instead, we have to send unmanned machines to do our dirty work so we can stay safe here on the surface.
While there are animals that can live at the deepest depths of the ocean, they have squishy structures, so the pressure doesn't bother them that much. There may be tons of undiscovered animal species down there, and we won't know until we really explore the remaining 90-95% of our oceans.