Avatar the Last Airbender TP Volume 7 (Rift Part 1)

Avatar Aang asks his friends to help him honor Yangchen's Festival-one of the highest Air Nomad holidays, which hasn't been celebrated in over one hundred years. But 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 a more mysterious menace afoot?