In this episode, the spotlight is on war, and our journey goes from the frontlines of World War II tunnels. As a legacy of a global conflicts, Europe is literally riddled with holes. Perhaps the most dramatic example of this is found in the Dutch town of Maastricht, which sits atop over 200 miles of tunnels and chambers. A descent into them is a trip back to the dark days of the Nazi occupation. In Switzerland, military tunnels and hidden rooms are still very much in use. In fact, the Swiss Alps conceal a maze of caverns and passages large enough to house the entire Swiss military.