This category gathers historical underground sites, including industrial mines, military counter-mine galleries, and subterranean mills. It covers the extraction of coal, iron, salt, and slate across Europe, from the medieval origins of Wieliczka to 19th-century sites like the Guido mine and the Blue Mine in Anjou. Readers discover how subterranean engineering evolved over centuries to exploit natural resources, establish post-war mills in Malta, and construct defensive military networks beneath fortifications.