The Ancient One by T.A. Barron presents an ecologically-minded Girls Underground story. Thirteen year-old Kate comes to stay with her great aunt in the woods of Oregon, and becomes entangled in the aunt’s struggle to save an ancient redwood forest which lies in a crater, from the loggers who would destroy it. When Kate foolishly separates from her aunt and is threatened by the loggers, she takes an action that thrusts her hundreds of years into the past, when the same world is populated by natives and fairy creatures, and threatened by the dark spirit of a volcano.
Her guide is a young native girl, and she is also accompanied by a fairy create and an unwilling human. She must confront the evil volcano god (alone) in time to save their world as well as return to her own time and make sure her aunt is safe from the loggers.
(Another book featuring the same protagonist, Heartlight, deserves an “honorable mention” credit as a GU story.)