What is located on the bottom in the water outside of Birka? Remains of a water palisade or jetties and other constructions.

Birka a Viking Age town that existed between AD 750 and 975 was located on the northwestern part of the small island of Björkö, in the Mälar archipelago of the Baltic Sea in Sweden. The Town was protected onshore by a hill fort and a town rampart. It is a widely spread assumption that Birka had a water palisade as a part of its defense. There are logs and other remnants on the bottom of the lake dating back to the Viking age. Uncertainty remains as to the origin of these remnants. The questions being, are they from a water palisade or the remnants of jetties and other constructions. The amount of fire wood alone needed to support 500 inhabitants for one year is equal to a wall of wood one meter high, one meter wide and two kilometers in length. This calculation does not take into account the wood used for transportation of other materials, people and animals. The conclusion is that future examination of the area is necessary to find out what is located on the bottom in the water.