Tokyo is the second-largest urban economy worldwide by gross domestic product after New York City, and is categorized as an Alpha+ city by the Globalization and World Cities Research Network. It is also Japan’s leading business hub as part of an industrial region that includes the cities of Yokohama, Kawasaki, and Chiba. As of 2021, Tokyo is home to 37 companies of the Fortune Global 500.[10] In 2020, the city ranked fourth on the Global Financial Centres Index, behind only New York City, London, and Shanghai.[11] Tokyo is home to the world’s tallest tower, the Tokyo Skytree,[12] and the world’s largest underground floodwater diversion facility, the Metropolitan Area Outer Underground Discharge Channel (located in Kasukabe, Saitama, a suburb of Tokyo).[13] The Tokyo Metro Ginza Line, opened in 1927, is the oldest underground metro line in East Asia.[14] Recognized as one of the most livable cities in the world, Tokyo was tied fourth with Wellington in the 2021 Global Livability Ranking.