Board Thread:General Discussion/@comment-97.121.194.7-20130824223029/@comment-14027812-20130824223747

IT leads to the underground place called Blacksreach,and,from what i remember,you need the Attunement Sphere you get in the quest Discerning the Transmundane to open it.North of the College of Winterhold,there's a man named Septimus Signus living in a cave.You get the quest from him.

I could be very wrong,though.