Hạ Long Bay has an area of around 1,553 km2, including 1,960–2,000 islets, most of which are limestone. The core of the bay has an area of 334 km2 with a high density of 775 islets. The limestone in this bay has gone through 500 million years of formation in different conditions and environments. The evolution of the karst in this bay has taken 20 million years under the impact of the tropical wet climate. The geo-diversity of the environment in the area has created biodiversity, including a tropical evergreen biosystem, oceanic and sea shore biosystem. Hạ Long Bay is home to 14 endemic floral species and 60 endemic faunal species.
Historical research surveys have shown the presence of prehistorical human beings in this area tens of thousands years ago. The successive ancient cultures are the Soi Nhụ culture around 18,000–7000 BC, the Cái Bèo culture 7000–5000 BC[6] and the Hạ Long culture 5,000–3,500 years ago. Hạ Long Bay also marked important events in the history of Vietnam with many artifacts found in Bài Thơ Mount, Đầu Gỗ Cave, Bãi Cháy.
500 years ago, Nguyễn Trãi praised the beauty of Hạ Long Bay in his verse Lộ nhập Vân Đồn, in which he called it "rock wonder in the sky". In 1962, the Ministry of Culture, Sports and Tourism of North Vietnam listed Hạ Long Bay in the National Relics and Landscapes publication. In 1994, the core zone of Hạ Long Bay was listed by UNESCO as a World Heritage Site according to criterion vii, and listed for a second time according to criterion viii.
Located in Hạ Long and Bái Tử Long are archaeological sites such as Mê Cung and Thiên Long. There are remains from mounds of mountain shellfish (Cyclophorus), spring shellfish (Melania), some fresh water mollusc and some rudimentary labour tools. The main way of life of Soi Nhụ's inhabitants included catching fish and shellfish, collecting fruits and digging for bulbs and roots. Their living environment was a coastal area unlike other Vietnamese cultures, for example, like those found in Hòa Bình and Bắc Sơn.
Cái Bèo culture (5000–3000 BC)
Located in Hạ Long and Cát Bà island, its inhabitants developed to the level of sea exploitation. Cái Bèo culture is a link between Soi Nhụ culture and Hạ Long culture.
Hạ Long Bay has experienced at least 500 million years in various geological states of orogeny, marine transgression and marine regression. During the Ordovician and Silurian periods (500-410 million years ago), Hạ Long Bay was deep sea. During the Carboniferous and Permian periods (340-250 million years ago), Hạ Long Bay was at shallow sea level.
The dominated uplift movement of neotectonic and recent tectonic influenced deeply on topography of this area, and the present landscape of sea-islands was formed around 7 or 8 thousand years ago by the sea invasion during Holocene transgression begun at about 17-18 thousand years ago. Particularly from the Holocene time, from about 11,000 years ago Cat Ba - Hạ Long area has much archaeological evidence connecting variations in sea levels with the development of ancient cultures such as the Soi Nhu and Ha Long cultures.
Feudal period
History shows that Hạ Long Bay was the setting for local naval battles against Vietnam's coastal neighbors. On three occasions, in the labyrinth of channels in Bạch Đằng River near the islands, the Vietnamese army stopped the Chinese from landing. In 1288, General Trần Hưng Đạo stopped Mongol ships from sailing up the nearby Bạch Đằng River by placing steel-tipped wooden stakes at high tide, sinking the Mongol Kublai Khan's fleet.
During the Vietnam War, many of the channels between the islands were heavily mined by the United States navy, some of which pose a threat to shipping to this day.
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