The Thalassian Pass[48, 85] is located in the far southern Ghostlands, and is the passage through the mountains south to the Eastern Plaguelands. It is marked with a large portal that hinders non-BC owners from entering Quel'Thalas.
The large gatehouse near the portal is believed to be one of the Elfgates (probably the Outer Elfgate, judging by its proximity to the border) that sealed off Quel'Thalas from Lordaeron. The gates are flanked by Scourge banners - possibly placed when the Scourge snuck through Quel'Thalas' defenses, aided by the traitor Dar'Khan Drathir.
Seems to have been called Greenwood Pass in Warcraft III.