Why does this horizontal line appear across the model when I add a subsurf modifier? So I have a screenshot for you, of the rear end of my female 3D model, and there in a strange indent going through it. Which only appears when I use a subdivision surface modifier.

And no, I am not talking about here butt crack xD I am talking about the horizontal line that goes all the way around.

It is unique from the possible duplicate due to the fact that it is a horizontal line, and thus is not a mirroring issue.

Here is the image:
Go in edit mode delete the faces (but not the points) where there is the horizontal lines, and then create the faces again.