Bjørnsjøhelvete is a cool spot with local timber history. They used to float timber down on the river here. A hanging bridg spans the narrowest point of this river, which was quite dangerous for the workers, hence the name 'The Bjørnsjø Hell'. The hanging bridge makes a great destination for a hike in Nordmarka forrest area. As anything in Nordmarka, you can start anywhere, but I recommend getting off the bus stop Hammeren. From there, it's still a fairly long hike of maybe 10k. There are signs that make it easy to follow the right trail, but we still recommend having a map (for example the Ut.no map online). Certain parts can be a little steep, and due to its length it's recommended for people used to walking!
Come during May to October on foot, or by skis in the winter.