Many of them are still under construction. The D1 motorway connects Bratislava to Trnava, Nitra, Trenčín, Žilina and beyond, while the D2 motorway connects it to Prague, Brno and Budapest in the north–south direction. A large part of D4 motorway (an outer bypass), which should ease the pressure on Bratislava's highway system, is scheduled to open in 2020. The A6 motorway to Vienna connects Slovakia directly to the Austrian motorway system and was opened on 19 November 2007. Slovakia has three international airports. Bratislava's M. R. Štefánik Airport is the main and largest international airport. It is located northeast of the city centre. It serves civil and governmental, scheduled and unscheduled domestic and international flights. The current runways support the landing of all common types of aircraft currently used. The airport has enjoyed rapidly growing passenger traffic in recent years; it served 279,028 passengers in 2000 and 2,292,712 in 2018. Košice International Airport is an airport serving Košice. It is the second-largest international airport in Slovakia. The Poprad–Tatry Airport is the third busiest airport, the airport is located 5 km west-northwest of Poprad. It is an airport with one of the highest elevations in Central Europe, at 718 m, which is 150 m higher than Innsbruck Airport in Austria. Railways of Slovak Republic provides railway transport services on national and international lines. The Port of Bratislava is one of the two international river ports in Slovakia. The port connects Bratislava to international boat traffic, especially the interconnection from the North Sea to the Black Sea via the Rhine-Main-Danube Canal. Additionally, tourist boats operate from Bratislava's passenger port, including routes to Devín, Vienna and elsewhere. The Port of Komárno is the second largest port in Slovakia with an area of over 20 hectares and is located approximately 100 km east of Bratislava. It lies at the confluence of two rivers - the Danube and Váh. Tourism Slovakia features natural landscapes, mountains, caves, medieval castles and towns, folk architecture, spas and ski resorts. More than 5,4 million tourists visited Slovakia in 2017. The most attractive destinations are the capital of Bratislava and the High Tatras. Most visitors come from the Czech Republic (about 26%), Poland (15%) and Germany (11%). Slovakia contains many castles, most of which are in ruins. The best known castles include Bojnice Castle (often used as a filming location), Spiš Castle, (on the UNESCO list), Orava Castle, Bratislava Castle,