The Ancient Rusted Iron key is located at area #32 of Gamebanshee's map of Main Halls inside the ancient dwarven chest. They is bug with the key however, sometimes the chest is empty and you cannot get the key but patch 1.4 fixes that bug so it seems.
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"When I am working on a problem I never think about beauty. I only think about how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong."
Buckminster Fuller | 
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