Halong Bay covers a total area of 1,553km², including 1,969 islands of various sizes, 989 of which have been given names. There are two kinds, limestone and schist, which are concentrated in two main zones: the south-east (belonging to Bai Tu Long Bay) and the south-west (belonging to Halong Bay). The average geological age of the islands is between 250 and 280 million years old.