There is only one file type that uses the .TGO extension:
Terragen Object File
Object created by Terragen, a landscape generation program; contains a three dimensional object and any attached nodes; similar to the .OBJ file but in the Terragen native format; objects built into Terragen include grass clumps, landmarks, rocks, and spheres.
To import a TGO file, select, choose your save location, verify that the File Format is TGO, and click .
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Updated: February 9, 2014