Speak with Rabine Saturna in the village of Nighthaven, Moonglade. Moonglade lies between Felwood and Winterspring, accessible through a path out of Timbermaw Hold.
For this task, Keeper Remulos' majordomo in Moonglade - Rabine Saturna - seeks aid in exploring the vast wastelands far to the west of Tanaris... and even further west of Un'Goro. You should find him in Nighthaven, the main village of Moonglade.
While your work will no doubt be of tremendous benefit to us all, I advise you to tread gingerly. There are tensions between Darnassus and Moonglade... ones that may grow over the course of time.