Lonna

This guide to Lonna, the small island near Suomenlinna in Helsinki, traces its double life: a Russian-built mine depot from 1817 whose real business, once diesel generators arrived in 1944, was demagnetizing warships so they could pass over mines undetected. It follows the island through decades of military use, the generators falling silent in the late 1960s, and the navy's hold on it until 1999. A boat company and an advertising agency later took over the old mine sheds, turning them into a restaurant and, in 2017, a public sauna.