From Experimental Blog.
Lonna island is part of Suomenlinna, on Kruunuvuorenselkä. In the Finnish war the island served as a place for negotiations about Viapori. In the Russian period, buildings were built for the making and storage of sea mines.
During WWII and all the way to 1955, there was a demagnetization station for defending against magnetic mines. Metal war boats used to come to the island monthly and they were demagnetized in order to make them immune to magnetic mines (- Open House Helsinki 2012).