Halong Bay is made up of 1,969 islands of various sizes, 989 of which have been given names. 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.