Wailing Caverns

The Wailing Caverns in the Northern Barrens, to the south of Crossroads, on the north side of the Lushwater Oasis. The Wailing Caverns is one of the first instanced dungeons that lower-level Horde players encounter. One primary reason to venture into the Wailing Caverns is for BoP drops from the numerous bosses in the instance. Some of the quest rewards are also highly desirable. Even the poor quality drops are better than ordinary; a stack of typical trash drops is worth around five to ten silver.

History
From Blizzard's Official World Dungeons site:
 * In years past, a night elf druid named Naralex discovered a network of underground caverns within the heart of the Barrens. Dubbed the 'Wailing Caverns', these natural caves were filled with steam fissures which produced long, mournful wails as they vented. Naralex believed he could use the caverns' underground springs to restore lushness and fertility to the Barrens - but to do so would require siphoning the energies of the fabled Emerald Dream. Once connected to the Dream however, the druid's vision somehow became a nightmare. Soon the Wailing Caverns began to change - the waters turned foul and the once-docile creatures inside metamorphosed into vicious, deadly predators. It is said that Naralex himself still resides somewhere inside the heart of the labyrinth, trapped beyond the edges of the Emerald Dream. Even his former acolytes have been corrupted by their master's waking nightmare - transformed into the wicked Druids of the Fang.

Dungeon Journal
''Years ago, the famed druid Naralex and his followers descended into the shadowy Wailing Caverns, named for the mournful cry one hears when steam bursts from the cave system's fissures. Naralex planned to use the underground springs to restore lushness to the arid Barrens. But upon entering the Emerald Dream, he saw his vision of regrowth turn into a waking nightmare, one that has plagued the caverns ever since.''