Bangkok has been Thailand's dominant city since the eighteenth century. Established as the capital in 1767 after the fall of the ancient city of Ayutthaya, Bangkok is located on one of southeastern Asia's most important rivers, the Chao Phraya. In the 1980s and 1990s, Bangkok underwent rapid modernization, growing to approximately ten million people, making this the only major city in a country of villages and small towns. Bangkok's ancient palaces and Buddhist temples contrast with high rises and traffic jams. Dense air pollution blankets the city. In fact, police directing traffic are required to wear masks. Still, despite modern problems, Bangkok is a major tourist destination, drawing millions of visitors each year.