and over 33 million in 2018, making Greece one of the most visited countries in Europe and the world, and contributing 18% to the country's GDP. Athens welcomed more than 5 million tourists in 2018, and 1.4 million were "city-breakers"; this was an increase by over a million city-breakers since 2013. Transport Athens is the country's major transportation hub. The city has Greece's largest airport and its largest port; Piraeus, too, is the largest container transport port in the Mediterranean, and the largest passenger port in Europe. Athens is a major national hub for Intercity (Ktel) and international buses, as well as for domestic and international rail transport. Public transport is serviced by a variety of transportation means, making up the country's largest mass transit system. The Athens Mass Transit System consists of a large bus and trolleybus fleet, the city's Metro, a Suburban Railway service and a tram network, connecting the southern suburbs to the city centre. Bus transport OSY () (Odikes Sygkoinonies S.A.), a subsidiary company of OASA (Athens urban transport organisation), is the main operator of buses and trolleybuses in Athens. As of 2017, its network consists of around 322 bus lines, spanning the Athens Metropolitan Area, and making up a fleet of 2,375 buses and trolleybuses. Of those 2,375, 619 buses run on compressed natural gas, making up the largest fleet of natural gas-powered buses in Europe, and 354 are electric-powered (trolleybuses). All of the 354 trolleybuses are equipped to run on diesel in case of power failure. International links are provided by a number of private companies. National and regional bus links are provided by KTEL from two InterCity Bus Terminals; Kifissos Bus Terminal A and Liosion Bus Terminal B, both located in the north-western part of the city. Kifissos provides connections towards Peloponnese, North Greece, West Greece and some Ionian Islands, whereas Liosion is used for most of Central Greece. Athens Metro The Athens Metro is operated by STASY S.A () (Statheres Sygkoinonies S.A), a subsidiary company of OASA (Athens urban transport organisation), which provides public transport throughout the Athens Urban Area. While its main purpose is transport, it also houses Greek artifacts found during the construction of the system. The Athens Metro runs three metro lines, namely Line 1 (Green Line), Line 2 (Red Line) and Line 3 (Blue Line) lines, of which the first was constructed in 1869, and the other two