terrain-exporter-unity
A small plugin to allow terrains to be exported. This is an edit of a conversion of an original, where this one includes extra features (like flipping positions and exporting multiple terrains)
This edit of the other version allows you to export all terrains you have selected in the editor
Installing
Just drag and drop this file into your project, and unity should auto-detect it and you should see a "Terrain" menu below the title bar.
Click that and click "Export to Obj...", which shows some settings for exporting
Then click "Export" and select a folder, which is where your selected terrains will be saved to
Depending on how big and how high your terrain resolutions are, it may take some time to export. I exported 9 terrains in about 10 seconds, and each file was about 8.3mb on average