I have started to model a torso. I was happy with it, until I discovered the seam in the mid of the mesh. I have not used the mirror modifier at all, what else could it be?
1$\begingroup$ Not enough information. Could you add a screenshot in edit mode, and one of the modifier stack? $\endgroup$– Róbert László PáliAug 21, 2016 at 10:31
$\begingroup$ I edited the OP $\endgroup$– Pure RPGAug 21, 2016 at 10:49
It seems that half of the normals in your model are inverted (the normals of some faces are facing inward and some outward). This might be causing display problems.
Edit Mode, select all faces and use CTRL + N to recalculate normals.
If recalculating the normals doesn't help, display face normals as lines, select only the faces with normals facing inward, and flip normals on those faces.
$\begingroup$ Happy I could help :) $\endgroup$– KarolinaAug 21, 2016 at 13:31
$\begingroup$ You shold accept Karolinas answere. :) $\endgroup$ Aug 21, 2016 at 17:26
It looks like your main issue is that you have some geometry hidden in the middle. Then the subdivision modifier is taking those hidden faces into account and making the crease.
I would recommend trying the "Select Non Manifold" tool, found from the 3D view header Select > Non Manifold, or the very covenant keyboard shortcut CtrlAltShiftM. Then set it to only select the Boundaries in the operator properties part of the tool shelf.
But do not just delete what it selects, it will select the bottom and top loops of your mesh, you want to keep those.
If that does not help, try looking in wireframe shading, Z to toggle between solid and wireframe, for any gemotery that is not part of the outside mesh.
You also have half of the mesh's normals reversed (the dark part). Just select everything and press CtrlN to fix that.
More info here: How to make all faces flip to the right/consistent direction?