Bai Tu Long Bay in the context of "Halong Bay"

Hạ Long Bay or Halong Bay (Vietnamese: Vịnh Hạ Long, pronounced [vînˀ hâːˀ lawŋm] ) is a bay located in Northeastern Vietnam. The name Hạ Long means "descending dragon". The bay is administered by the cities of Hạ Long and Cẩm Phả in Quảng Ninh province. It features thousands of limestone karsts and islets in various shapes and sizes, for which it is listed as a UNESCO World Heritage Site and a popular travel destination. The bay is also part of a larger area that includes Bai Tu Long Bay to the northeast and Cát Bà Island to the southwest, with these zones sharing similar geological, geographical, geomorphological, climate, and cultural characteristics.

Hạ Long Bay has an area of around 1,553 km (600 sq mi), including 1,969 islets, most of which are limestone. The core of the bay has an area of 334 km (129 sq mi) 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 and a seashore biosystem. Hạ Long Bay is home to 14 endemic floral species and 60 endemic faunal species.