The transportation system of New York City is a network of complex infrastructural systems. New York City, being the most populous city in the United States, has a transportation system which includes one of the largest subway systems in the world; the world's first mechanically ventilated vehicular tunnel; and an aerial tramway. New York City is also home to an extensive bus system in each of the five boroughs; citywide and Staten Island ferry systems; and numerous yellow taxis and boro taxis throughout the city.