Ha Long Bay or Halong Bay is a UNESCO World Heritage Site and popular travel destination in Quảng Ninh province, Vietnam. The name Hạ Long means "descending dragon". The bay features thousands of limestone karsts and islets in various shapes and sizes. Hạ Long Bay is a center of a larger zone that includes Bai Tu Long Bay to the northeast, and Cát Bà Island to the southwest.

Hạ Long Bay has an area of around 1,553 km, including 1,969 islets, most of which are limestone. The core of the bay has an area of 334 km 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.

According to local legend, when Vietnam had just started to develop into a country, they had to fight against invaders. To assist the Vietnamese in defending their country, the gods sent a family of dragons as protectors. This family of dragons began spitting out jewels and jade. Under magics, numerous rock mountains abruptly appeared on the sea, ahead of invaders' ships; the forward ships struck the rocks and each other. These jewels turned into the islands and islets dotting the bay, linking together to form a great wall against the invaders.