What did I drill into?
If the magnetic stud finder is showing response across a good part of the stud cavity then there is probably a metal duct in the wall. This could be a stove hood vent, a dryer vent or a heating duct. 

If the magnetic finder is showing just a narrow range near where you drilled the hole then the item in the wall might be a metal pipe such as a sewer vent stack, an iron water pipe or an iron gas pipe. Since the first two examples would be pretty rare to use iron pipe I would put biggest odds on a gas pipe. 

There is another possibility that you ran into a metal electrical conduit.