From ancient Jerusalem to today's urban battlefields in Ukraine and Gaza, we can find many examples of urban conflicts that have been heavily influenced by their underground components. But how should military forces categorize subterranean spaces and consider the different functions of underground structures? How does this subterranean dimension influence the conduct of military operations? What about the unique hazards of sending soldiers underground? And what are some of the planning considerations for attacking or defending subterranean positions? This episode addresses those questions and more in a comprehensive overview of underground warfare.