Situated in the north-east region of Vietnam, Hạ Long Bay is a part of the Gulf of Bắc Bộ, and comprises Hạ Long City, the township of Cẩm Phả and a part of the island district of Vân Ðồn. To the south-west it borders the island of Cát Bà, to the east is the sea and the mainland follow a coastline of 120 km. It stretches between the 106º58 and 107º22 eastern meridians and the 20º45 and 20º50 northern parallels.
Topography
Hạ Long Bay covers a total area of 1,553 sq. km, 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 Bái Tử Long Bay), and the south-west (belonging to Hạ Long Bay). The average geological age of the islands is between 250 and 280 million years old.
The densely concentrated zone of stone islands, grottoes and caves, world famous for its spectacular scenery, forms the central zone of Hạ Long Bay, which has been listed as one of UNESCO’s World Heritage Sites. This protected site covers an area of 434 sq. km, comprises 775 islands and forms a triangle: with the Ðầu Gỗ Island (Driftwood Island) to the west; the Ba Hầm Lake (Three Shelter Lake) to the south; and the Cống Tây Island to the east. The regions immediately surrounding the area were classified as a national site by the Ministry of Culture and Information in 1962.
Climate
Hạ Long Bay is located in a tropical and temperate zone. The four distinct seasons are most evident in a year. The annual average temperature is 22,8ºC. The average temperature in summer is 26,4ºC and the hottest temperature is 35,7ºC. The annual average rainfall is 2,005.4 mm. The period from May to October receives the more important rainfall. The winter lasts from 4 to 5 months. Between the main two seasons are a shorter spring and autumn. The period from August to October is typhoon season.
History
Hạ Long Bay has been called by the great national poet Nguyễn Trãi: "a wonder of the earth erected towards the high sky.” It is also a place closely linked to Vietnam’s history with such famous geographical names as: Vân Ðồn (site of an ancient commercial port); Poem Mountain (with engravings of many poems by emperors and other famous people of the past); and Bạch Ðằng River (the location of three fierce naval battles fought against foreign aggressors).
This is not all; Ha Long has been proven by scientists to be one of the first cradles of human existence in the area, with such archaeological sites as Ðồng Mang, Xích Thổ, Soi Nhụ and Thoi Giếng. It is also a region of highly-concentrated biological diversity with many varied ecosystems of salt water-flooded forests, coral reefs and tropical forests, featuring thousands of diverse species of animal and plant life.
While exploring the bay, it’s hard not to feel lost in some legendary world of stone islands. There is an island resembling a man standing and looking towards the mainland. Dragon Island looks like a dragon hovering above the turquoise water. Yet another island, Lã Vọng, resembles an old man fishing, and was named after a famous Chinese mandarin who abandoned his position to become a fisherman. There are also the islands of the Sail, the Trống Mái Islet (Cock and Hen Islet), which look like a pair of chicken lovingly playing with each other above the sea; and the Ðỉnh Hương Islet (Incense Burner Islet), which all bear astonishing resemblance to their namesakes. The forms of the islands change incessantly, depending on the angles of lighting and from where they’re viewed.
At the core, are wonderful caves and grottoes such as: Thiên Cung Grotto (Heavenly Residence Grotto), Đầu Gỗ Grotto (Driftwood Grotto), Sửng Sốt Grotto (Surprise Grotto) and Tam Cung Grotto (Three Palaces Grotto).
With such special values, at the 18th Session of UNESCO’s Council of World Heritage held on 17 December 1994 in Thailand, Hạ Long Bay was officially placed on the list of the World Natural Heritage. In 2000, UNESCO recognized it as the World Heritage for the second time for its geographical and geomorphologic values. This confirms the global premier value of Hạ Long Bay.

Source: halong.com