The tunnel running under the English Channel -- also known as the Chunnel -- is one of only a few international tunnels and provides a direct rail link between London and Paris.

The tunnel itself is actually three tubes -- two for trains and one for service vehicles. It's 31 miles long, with an average depth of 50 feet below the seabed. Its fastest trains hit 186 miles per hour.

Though it opened in 1994, the idea was a long time coming. It was discussed by Napoleon in 1802, who proposed a tunnel with a mid-channel island "to breathe the horses."

Over the next two centuries, several attempts were made to start construction, although the British often got cold feet. "You ask us to contribute to a work the object of which is to shorten a distance which we already find too short," quipped one 19th century British politician.

A 1959 proposal pegged the tunnel's price at $100 million. The final tab was $15 billion.