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.
The Ironmarch took control of the upper Blackrock Spire within the mountain and dug a passage to the Molten Span. They erected a platform where they intended to prime one of their Iron stars which could act as a weapon of mass destruction capable of erupting an entire volcano.
In the RPG
The Molten Span is a connection between the Burning Steppes and the Searing Gorge located within the Blackrock Spire volcano. This massive stone bridge crosses the gorge of fire created by Ragnaros, and it precedes the fortress of Blackrock Spire. It is a fine construction indeed. 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.