"Subterranean warfare—whether involving human-made tunnels, underground urban infrastructure, or natural cave networks—has been an element of U.S. military operations from the Second World War and Vietnam to Iraq and Afghanistan. As above-ground commercial and military intelligence, surveillance, and reconnaissance (ISR) capabilities continue to grow more capable and ubiquitous, adversaries are increasingly heading underground to circumvent detection." -- Homeland Security News Wire, November 27, 2017.
Secret Underground Cities: An Account of Some of Britain's Subterranean Defence, Factory and Storage Sites in the Second World War. by