Sudden loss of terrain after import (U4.0/MacOSX)

I am working on a scene, and imported a friend's scene into it because I wanted to look at some of her scripts. It looks like it replaced my terrain data with the terrain data from the imported scene (they were both called 'terrain'). I have just restored my whole project file from a backup that I made previous to the import, restarted Unity and the terrain is still the imported terrain! Is terrain data stored somewhere outside of the project file, in a library or something!?!