There is a train into that level that tourists can take and have a guided tour.
There are two main levels in kongsberg, underbergstollen and overbergstollen. The way it works is that many mines go from the top and end up in one horisontal tunnel. lot's of them continue below this level but they are all connected. underbergstollen is about 6 km long if i remember correctly. Overbergstollen is the same kind of thing but i have not been there and i don't know how long it is.
The mine we were in goes into underbergstollen and by chance we met a load of tourists being guided. The mine is much deeper than this but is waterlogged from underbergstollen and downwards.
There is also a hall that is used for large dinners like company xmas partys in kongens gruve (where the train goes).
Pics:
There is also a film from the mines
here that shows the train etc.
Some info about kongsberg and it's mining industry in english
If you have any more questions, don't hesitate to ask!
Francis