Tunnels play an important part in our lives. They are big, often circular, tubes that run underground. To be called a tunnel, they usually have to be big enough for people to get through. Most of us live with tunnels underneath our feet: they are sewer tunnels.

We also use tunnels to carry railways and roads under rivers or even the English Channel. We use tunnels to get to valuable minerals such as coal and gold.

The world's biggest tunnel system is the London Underground.