Gothenburg's secret, steel-plated warship Sölve

Sölve, a Hildur-class monitor, was the first of its kind to float. Launched in 1875 from Motala Verkstad at Norrköping, it weighed 480 metric tons and could reach speeds of 8 knots. Its crew of 48 soldiers was ready for action for over a century. However, by 1919, oil had made these great ships obsolete, and Sölve was decommissioned and sold. For ninety years, it has lain silent in Gothenburg, one of the city's surprising treasures. Today, the Swedish Maritime Centre has restored Sölve, and she looks ready for action, bobbing on a mooring in the Maritiman museum.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons