How can I make a cave with Unity? I know some people are saying that I can’t. But if I can’t will somebody please give me a free model made on Blender or a tutorial to make one on blender?
Use the script on this website to convert your completed terrain to an model. With that you can edit your terrain in another program for making caves 
.
What kind of script is it?
It is js. It is used by the Unity engine and executed from there.
Wait, can I use the script to make a cave right in Unity or do I have to export a cave and edit that.
No, you use this script to export your terrain as a model. You then edit that model in a program and make a cave. Once done you import the model of the terrain with a cave back into Unity. I don’t think you can convert it back to a terrain so make sure you are done the terrain.
Can I use blender?
Yup.
I tried it and my Unity says that it needs to be outside the editor folder and then it just shuts down.
:?
Odd, maybe someone with experience with the converter can help? Calling all unity people.