Just south of the old church in the middle of Oslo lays an old silver mine. Personally I've been living in Oslo for 15 years, and probably been sitting and drinking a beer just 10-15 meters away from the silver mine without knowing it even excisted. This is a very exciting area, with a lot of old houses in Akerbakken. The church it self is as old as christianity itself in Norway.
This is supposedly the oldest mine in Norway, and in "historia norvegiae" there is written about a cave in Oslo called "the dragon cave". It also says it is rich of silver. The mine was shut down in the late 11th century because of water. It was probably operative around the time Harald Hardråde founded Oslo in 1050.
Later during the mid 16th century german miners ran the mine. The mines was lensed to about 40 meter depth, but since it went deeper than the Oslofjord there was always a problem with water.