Halong Bay is located in the Gulf of Tonkin and comprises approximately 1,900 islands and islets. There are two kinds of islands, limestone and schist, which are concentrated in two main zones: the southeast (belonging to Bai Tu Long Bay), and the southwest (belonging to Halong Bay). This densely concentrated zone of stone islands, world famous for its spectacular scenery of grottoes and caves, forms the central zone of Halong Bay, which has been named a UNESCO World Heritage Site. According to the legend of its creation, the gods sent a family of dragons to help defend the Vietnamese against the Chinese invaders. The dragons descended upon what is now Ha Long Bay (hence the name Bay of Descending Dragons) and began spitting out pearls. The pearls turned into huge rock islands which formed a barricade against the Chinese ships, which crashed and sank.