How do I make both sides of a plane/mesh visible? I've made a map for my game which the player can run about and also enter a cave, however the issue is that when I have the faces facing out, the cave becomes invisible and when I flip the faces the other way (using the ctrl+shift+n method in editing in blender) it makes the top/surface of the map invisible, showing the bottom of the map.

I would like to know how to make both sides of the mesh visible in Unity or blender to avoid this issue and due to copyright I cannot upload the file as it's for an upcoming game.

I'm asking this because everything I've looked at and tried hasn't worked for me :(

This is a screenshot to show what happens when I try backface culling:
In Blender you can turn on Backface Culling in the shading tab of the Properties sidebar of the 3D View.

In Unity you need a special shader that can do Backface Culling. This feature is not transferable. Or you simply double the geometry, and flip the normals for one of the meshes then.