A file with .eta extension is a Google Earth Placemark file that is created with Google Earth software. It contains bookmarks/marker information for a location in the world using parameters such as latitude, longitude, rotation, zoom, and title. This enables users to have a bird-eye view of the location. With the introduction of the new file formats i.e. .kml and .kmz, the ETA file format was phased out and was replaced with these new file formats. ETA files can be opened in Google Earth software that is available for download free of cost.
ETA File Format - More Information
Internal details of ETA file format are not available publicly and there are no open-source or proprietary software available to read the information ETA files.