Or the bottom of a lake. Or a well. Or a cave? At the bottom of a body of water. But that sounds a lot less interesting. 

We don't know much about the oceans, collectively. We never have. Physical forces make the glimpses we get of the depths brief and expensive. It is no wonder, then, that we see all sorts of terrors in the deep. 

Realistically? The ocean is just as uncaring about our existence as the rest of creation. But that won't stop things like this from being created.