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The Øresund is a cable-stayed bridge that runs nearly 5 miles (8 km) to an artificial island where it transitions into a tunnel that runs another 2.5 miles (4 km). It was designed by Danish engineering firm COWI and connects the Danish capital of Copenhagen to the Swedish city of Malmö.

This spectacular dome with an underground tunnel is no longer in use, even though it sits on a privately owner estate. Built in 1890, the underground space was used as a ballroom and billiard or smoking room.

This is the Gotthard base tunnel which is almost 36 miles long. It is the longest railway tunnel in the world. It runs underneath the Alps in Switzerland and is scheduled for opening in 2017. The breakthrough to the east tube just happened in October of 2010.

If you want to know more about the tunnel, check out the article in wikipedia about it.