Avatar: The Last Airbender - The Rift

Avatar: The Last Airbender - The Rift summary:

Avatar Aang asks his friends to help him honor Yangchen's Festival - an extremely sacred Air Nomad holiday that has not been held in over a hundred years. However, cryptic visits from the spirit of Avatar Yangchen herself lead Aang to discover a jointly owned Fire Nation and Earth Kingdom refinery - operating on land sacred to the Airbenders! Is this the cause of the pollution Aang finds there, or is something more mysterious happening?