Sun Light Bleeding Inside Enclosed Mesh?

I modelled a cave mesh with no holes except the entrance but light still bleeds in the inner parts of the mesh where it is pitch dark.

By bleed, I mean tiny bits of light only leaks into the inside of the mesh.

I even set the mesh to double sided.

the sun lamp rotates in the x axis to simulate day and night cycle. The mesh is fully enclosed from top to bottom so there is no hole for the sun light to pass through other than the opening and I set the shadow to very hight resolution, four cascades, close fit and shadow distance to 100 only. Shadow near plane is set to 0.2
Any suggestions are appreciated


Demo video