The islet of Lonna is situated between the larger fortress island of Suomenlinna and the mainland of the city of Helsinki.

For much of its history, this tiny island served as a naval outpost. In the 19th century, Lonna was home to production and storage facilities. Up until 1955, it served as a demagnetizing station. Floating magnetic mines were a devastating weapon of maritime warfare against ships with metallic hauls. The demagnetization method was developed as a way to counter these mines. The process itself involved wrapping the vessel with electric cables and running the high-powered current for about six to eight hours. During World War II, Finnish and German warships would stop by once a month for this procedure. The diesel generators used for this purpose can still be seen on Lonna.

The island of Lonna was turned to civilian use in 2014. The old military structures have been converted into a restaurant, cafe, conference venue, and of course a sauna.