The speaker is referring to the natural dwelling-place of the snake. He slithers out of his home in a black hole to drink at the water trough, which is where he encounters the speaker, standing there in his pajamas, waiting to fill his pitcher. The black hole is described as a "dark door," the dark door of the "secret earth." The snake's home is secret because it's a part of the earth that humans seldom get to see. It's a dark, cavernous place that leads to the depths of the earth. The contrast between the snake's natural habitat and the human world above really couldn't be more stark. Above ground, all is visible; all is clear. But the same cannot be said for the subterranean world inhabited by the snake. There, all is dark and mysterious, and the secrets of this world wait to be discovered by man.