The Orgrimmar Tunnels are a series of tunnels that run underground Durotar, starting outside of the orc city of Orgrimmar. Gazlowe and his goblins dug them out in search of a reliable water supply for the Horde's new city. However, they mined directly to a kobold den, which put a halt to the operation. The kobolds were later routed by Rexxar and his companions, and the breaches into the dens were closed off, after which Gazlowe and his employees could finish their task. Rexxar also encountered an isolated group of wildkin within the earthen passageways.
Within the tunnels below Orgrimmar, the goblins have dug deep into the ground in search of water for the orcs' new home. Though the goblins are master diggers, they have little regard for the repercussions of turning so much soil.
The Tunnels were likely dug closer to Orgrimmar than the Southfury River, the closest external water source. The entrance to the Tunnels may even be within the city proper, as Orgrimmar was still under construction when digging began. Orgrimmar currently has three bodies of water within its limits; a waterfall in the Valley of Honor. and with the Cataclysm, a spring erupting from the cliffs above the Valley of Wisdom and another in the Valley of Spirits. None of these sources drain out of the city, which most likely indicates the underground water source sought by Gazlowe does indeed run under the city proper. The Tunnels may connect to the underground ruins of Gazlowe's Excavation Site, or to other caves in Durotar (even the volcanic Ragefire Chasm beneath Orgrimmar).