Commercial cruises between major coastal and port cities in the Baltic region, including Helsinki, Turku, Mariehamn, Tallinn, Stockholm, and Travemünde, play a significant role in the local tourism industry. There are also separate ferry connections dedicated to tourism in the vicinity of Helsinki and its region, such as the connection to the fortress island of Suomenlinna or the connection to the old town of Porvoo. By passenger counts, the Port of Helsinki is the busiest port in the world after the Port of Dover in the United Kingdom and the Port of Tallinn in Estonia. The Helsinki-Vantaa International Airport is the fourth busiest airport in the Nordic countries in terms of passenger numbers, and about 90% of Finland's international air traffic passes through the airport.