Situated in the center of the Paris basin (see Île-de-France), and only 90 mi (145 km) from the English Channel, the city handles a great volume of shipping. Orly and Charles de Gaulle airports (the latter opened in 1974) and many major railroad stations make Paris one of the great transportation centers of western Europe. The Paris metro (subway), built in 1900, was modernized and extended during the 1970s. There are now 16 principal metro lines and a high-speed express subway system servicing the suburbs. The system's hub, Chatelet Les-Halles, is perhaps the largest, busiest underground station in the world. Paris is also the hub of the national rail system, with high-speed trains connecting it to most major European cities.