Chittagong plays a vital role in the Bangladeshi economy. The Port of Chittagong, one of the world’s oldest ports, whose coast appeared on Ptolemy’s world map, is the principal maritime gateway to the country. The port is the busiest international seaport on the Bay of Bengal and the third busiest in South Asia. The Chittagong Stock Exchange is one of the country’s two stock markets. Several Chittagong-based companies are among the largest industrial conglomerates and enterprises in Bangladesh. The port city is the largest base of the Bangladesh Navy and Bangladesh Coast Guard; while the Bangladesh Army and Bangladesh Air Force also maintains bases and contributes to the city’s economy. Chittagong is the headquarters of the Eastern Zone of the Bangladesh Railway, having historically been the headquarters of British India’s Assam Bengal Railway and East Pakistan’s Pakistan Eastern Railway. A controversial ship breaking industry on the outskirts of the city, which supplies local steel but causes pollution, has come under international scrutiny.