How to set up a cave in Unity ?

I made a cave in blender and than imported it to unity but I realized that faces not visible from inside.

Why?
What should I do now ?

@Lili-Shi Just invert the normals in blender so they all face inwards. Then re import. The normals are basically the direction each face is pointing in. By default in blender they face outwards but you can switch them by selecting the appropriate faces and clicking to flip normals.

Well, either you force the mesh to have double-sided faces (which I don’t recommend, because it’ll look god-awful, or you create a mesh, which also has faces inside (which I would recommend, because you will have more control over the shape and the texture).