The Pearl River Delta (PRD), Zhujiang Delta or Zhusanjiao in Guangdong province, China is the low-lying area surrounding the Pearl River estuary where the Pearl River flows into the South China Sea. It is one of the most densely urbanised regions in the world and one of the main hubs of China's economic growth. This region is often considered an emerging megacity. The PRD is a megalopolis, with future development into a single mega metropolitan area, yet itself is at the southern end of a larger megalopolis running along the southern coast of China, which include large metropolises like Chaoshan, Zhangzhou-Xiamen, Quanzhou-Putian, and Fuzhou. The nine largest cities of PRD had a combined population of 57.15 million at the end of 2013, however their annual population growth rate is modest at 0.45%, comprising 53.69% of the provincial population. This sharply contrasts with the previous decade's migrant population boom, and reflects the rising cost of migrant labor and changes to the value chain. According to the World Bank Group, the PRD has become the largest urban area in the world in both size and population.