Exploring the forgotten underworld hiding beneath the streets of Cincinnati, USA. Late 19th Century Cincinnati was a hub of German immigrants who introduced lager to the United States. To make their beer they needed cool temperatures, so they headed underground.Also, a state-of-the-art Cold War fortress on the shores of Denmark. These dark tunnels were once the first line of defence for NATO in their battle against Russia and her allies. The clifftop guns above the caverns packed enough firepower to hold the Russian Baltic fleet while reinforcements arrived.And, there's a deep dive into the tunnels spanning the Mersey River in Liverpool. Driven by the rise of the automobile and increasingly clogged ferry routes, the Queensway Tunnel was both a feat of engineering and a world first when it opened in 1934.