The Energiebunker in Hamburg-Wilhelmsburg used to be an abandoned and overgrown flak and air raid bunker—a relic of World War II. After the war, the interior of the bunker was blown up, and only the upper floors and the three- to- four meter thick concrete walls remained. For more than 60 years the building was not in use. The building was transformed during the International Building Exhibition in Hamburg between 2006 and 2013 and is now used to generate renewable electricity and heat for more than 1500 households. This adaptive reuse not only enabled the bunker to be integrated into the redesigned neighborhood, but also to fulfill certain societal needs. The historic building was conserved and given a new sustainable use. Nowadays it is a tourist attraction and a war memorial. This paper explores the economic and environmental sustainability of this transformation. Could this be a second use scenario for other bunker facilities from World War II?