What man can lie motionless at the bottom of an empty swimming pool, without food or water, for who knows how long? What man can create music by waving his hand? What man can live inside a mirror?

More importantly:

The voice on the tape says something like, “I am alone, I am not alone. Someone, something, is here with me.” He talks about a sound, “the sound of nothing.” (Sound plays a very big role in this movie.) We hear that sound – one part howl, one part roar – and he says something like, “There. Did you hear it? That’s the sound.” The Man recognizes the sound, and he associates it with the disappearance of the world. To The Man in the Pool, he says something like, “I don’t know what happened. I don’t know what happened. It was just there, and then it was all gone. It happened here, too, didn’t it? But what? What was it? What is that sound?”

Later the voice on the tape says something like ” … you must run.” Filling in the blanks, it’s “If you hear that sound, you must run.”

Near the end of the movie, The Man rounds a corner and sees The Man in the Pool at the end of a hallway. The Man in the Pool wails the sound of nothing, the sound on the tape, the sound The Man associates with the vanishing of the world. (That was one of the creepiest, most frightening moments I’ve seen in any movie. Kudos to Doug Jones.)

So my conclusion is: The Man in the Pool is what happened to the hotel. He creates the sound that leaves nothing.

Of course the movie is about the The Man and The Woman and how their relationship is challenged and transformed by the hotel and The Man in the Pool. The sci-fi elements are secondary, and deliberately opaque.