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.
Warlords of Draenor
This section concerns content related to Warlords of Draenor.
The Ironmarch have taken control of the upper Blackrock Spire within the mountain and dug a passage to the Molten Span and 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 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.