In the summer, a secretive museum is open to the public: The Theta Museum
may be difficult to track down, but that is the point, really: this free one-room museum with a maximum capacity of 15 visitors at a time served as the headquarters for the Theta Group, who worked to keep up a radio connection between Norway and London during the Nazi occupation of Norway (1940-1945). The room was in use for two years until the Germans discovered and destroyed it in 1942. It was restored in the 1980s with the help of survivors from the resistance group, who were between 18 and 22 at the time. The museum is dependent on volunteers, opens at different times, and part of the museum is only accessible via some steep stairs. Those in the Bryggen area, head to Enhjørningsgården and give it a best shot.
Finally, those planning on visiting more than one museum, check whether a combination ticket may work—several of the museums in Bergen are run by the same organisations (e.g. the Hanseatic, Bryggen, and Fishery Museums), so those hoping to check out multiple museums may be able to pick up a discount.