Bolærne: Island Fortress in the Oslofjord

Bolærne is a small archipelago of three main islands and about forty skerries in the outer Oslofjorden, east of Nøtterøy. Fishermen used the islands from the 1500s, but in 1916 the military took over and built a coastal fortress that remained active through both world wars until it was decommissioned in 2002. Bunkers, trenches, and gun emplacements are still scattered across the islands and can be explored on foot. Guided tours of the fortifications are available in summer. The islands are now part of Færder nasjonalpark and accessible by boat.