Note: The Underforge only becomes accessible by joining the Companions during The Silver Hand from then onward.
It is accessed by a concealed stone door at the base of the Skyforge and has a alternate exit that leads outside the walls of Whiterun. This exit is strictly one way as it ends in a drop that is too high for the Dragonborn to jump up.
It is in this chamber that the ceremony for induction into the Circle is performed. The ceremony consists of two members of the Circle, one to recite an oath and another in wolf form to provide the Beast Blood.
The chamber also contains three altars for the Totems of Hircine.