Back in the day people built a lot of underground tunnels. They used it them mostly for escaping from a city. Rulers needed a quick exit in case of danger.
Do you know how many of these tunnels exist today? And where are they located? This documentary takes a look at some of the most amazing and fascinating underground tunnels in the world.
What are they being used for nowadays? Watch the video to see. An ancient tunnel system may have spanned the entire Earth, from one continent to another.
This might seem hard to believe, but legends and tales of subterranean worlds scattered throughout old texts.