Is there really stuff behind waterfalls?

In movies, video games, books…there’s always the classic case of waterfalls concealing something. Treasure, a secret entrance to some old ass ruins, stuff like that. I think we’ve all seen that at least once.
But is this idea based off of anything? Did someone in real life ever actually find something behind a waterfall? Are there any records of this happening? What was found?
Also, if it was a very large waterfall, wouldn’t the pressure injure and/or kill you if you attempted to pass through?