Hide part of mesh

I have problem with my geometry in Unity. Is there way how to hide just part of geometry ? Lets say i have a box and game level is inside that box, but i realized i need to remove one side of that box. Is there a way how to hide this polygon in Unity ?
Maybe with occlusion culling or somehow with mesh filter. I am lost here, but it would save me a lot of trouble to hide some geometry inside of Unity. I have cave and there are entrances to other parts of cave, but they are closed. Now we deciced we need open entrances to other parts of cave, but it would mean to recreate all caves, while i would rather just hide somehow the geometry where the entrance is, so that you can see to other part of the cave. So all i need is to hide somehow this part of the model of cave.
I hope i made it clear. It might be a bit confusing.

Thanks for any help.


While your question is a bit confusing, I think I got the gist of it. The simplest way to have these parts of your mesh disappear would be to model it separately. So, instead of trying to hide individual polygons in your huge mesh, you make the barriers separate gameObjects that you can then hide of animate away at will.

If you have to have a closed environment (as with your box example), you could play around with the near and far clipping planes on your camera.