The Molten Span is located inside Blackrock Mountain. It is the large central chamber with lava in the bottom and a central rock chained to the sides.
Lands of ConflictThis section concerns content exclusive to the Warcraft RPG, and thus is not necessarily canon.
The connection preceding the fortress of Blackrock Spire is a fine construction indeed. This massive stone bridge crosses the gorge of fire created by Ragnaros. If you do not wish to dangle your life high above a pool of lava, you may walk down stairs carved into the rock to an obsidian platform that runs all the way around the inside of the gorge, allowing for travel to another staircase on the other side. You will pass several massive statues of dwarves along the way, anchoring adamantine chains that suspend a great wedge of earth over the lava flow.