There is only one file type that uses the .UTW extension:
Unity Whirld File
World file created in the Whirld Project format, an open file format created for games developed with the Unity engine; contains a 3D world and related game assets; differs from the standard Unity package format so that other tools can create and edit game content.
UTW files can be saved and loaded in Unity after the Unity Whirld library has been added to the software. While some of the world features are supported in UTW files, some others may not be, including meshes and textures.
The first game developed using UTW files was Mars Explorer, which has since been succeeded by Syn3h.
NOTE: The UTW file extension was replaced by the .WHIRLD extension in Whirld 4.
Updated: June 25, 2013