With close to 220,000 inhabitants, Aberdeen is Scotland's third most populous city. Locally quarried grey granite was used during the mid-18th to mid-20th centuries for many of Aberdeen's buildings, and hence the nicknames it has earned as the Granite City, or the Grey City. Aberdeen granite was also used to build the terraces of the Houses of Parliament and Waterloo Bridge in London. Since the discovery of North Sea oil in the 1970s, Aberdeen has also been called the Oil Capital of Europe or the Energy Capital of Europe. It is no wonder that because of the oil fields in the North Sea, Aberdeen's seaport is very important. The Heliport with its flights to the oil fields is one of the busiest commercial heliports in the world.