Like many other cities, Helsinki was deliberately founded at a location on the sea in order to take advantage of shipping. The freezing of the sea imposed limitations on sea traffic up to the end of the 19th century. But for the last hundred years, the routes leading to Helsinki have been kept open even in winter with the aid of icebreakers, many of them built in the Helsinki Hietalahti shipyard. The arrival and departure of ships has also been a part of everyday life in Helsinki. Regular route traffic from Helsinki to Stockholm, Tallinn, and Saint Petersburg began as far back as 1837. Over 300 cruise ships and 360,000 cruise passengers visit Helsinki annually. There are international cruise ship docks in South Harbour, Katajanokka, West Harbour, and Hernesaari. In terms of combined liner and cruise passengers, the Port of Helsinki overtook the Port of Dover in 2017 to become the busiest passenger port in the world.