The tunnels of the world are a fascinating story.
But in reality they are an amazing sight.
Al Jazeera spoke to experts to find out more about what we see and hear on these fascinating journeys.
First, a look at the tunnels.
Alias A tunnel is a structure that can be built into a rock wall to provide a passage between two locations, like a bridge.
There are many different types of tunnels in a city.
They can be underground, underground rail tunnels, underground tunnels, subway tunnels, and so on.
They range from the very simple to the quite complex.
Here are some of the more common types of underground tunnels.
Underground rail tunnels