Hy all!
How can I make a cave that looks realistic, and I can walk into it?
You don’t. You make a cave in a 3d software. Lookup 3ds max/maya/blender/modo. And then uv wrap it and do the textures in photoshop/gimp whatever you like and import it into Unity. As for the walking part, import the package character controller and there’s a first person prefab for you to use.
Thanks for the reply.
That’s ok, that how can I create a hole that looks like a cave, but how can I make the terrain “around that”? To make a feel like I go into a cave that is inside a mountain.
Import the hole and then use the Unity terrain tools to sculpt around it 
I thought that won’t be so easy, but I will try that.
And how can I change to the opposite of the faces direction? I mean turn inside out. This is important to see the cave’s inside, not the outside.