The bay consists of a dense cluster of some 1,600 limestone monolithic islands each topped with thick jungle vegetation, rising spectacularly from the ocean. Several of the islands are hollow, with enormous caves. Hang Dau Go (Wooden Stakes cave) is the largest grotto in the Ha Long area. French tourists visited in the late 19th century, and named the cave Grotte des Merveilles. Its three large chambers contain large numerous stalactites and stalagmites (as well as 19th-century French graffiti). There are two bigger islands, Tuần Châu and Cát Bà, that have permanent inhabitants, as well as tourist facilities including hotels and beaches. There are a number of beautiful beaches on the smaller islands.
A community of around 1,600 people live on Hạ Long Bay in four fishing villages: Cua Van, Ba Hang, Cong Tau and Vong Vieng in Hung Thang ward, Ha Long city. They live on floating houses and are sustained through fishing and marine aquaculture (cultivating marine biota), plying the shallow waters for 200 species of fish and 450 different kinds of mollusks. Many of the islands have acquired their names as a result of interpretation of their unusual shapes. Such names include Voi Islet (elephant), Ga Choi Islet (fighting cock), Khi Islet (monkey), and Mai Nha Islet (roof). 989 of the islands have been given names. Birds and animals including bantams, antelopes, monkeys, and lizard also live on some of the islands.
Almost all these islands are as individual towers in a classic fenglin landscape with heights from 50–100 metres (160–330 ft), and height/width ratios of up to about six.
Another specific feature of Hạ Long Bay is the abundance of lakes inside the limestone islands. For example, Dau Be island has six enclosed lakes. All these island lakes occupy drowned dolines within fengcong karst.
Hạ Long Bay is located in northeastern Vietnam, from E106°55′ to E107°37′ and from N20°43′ to N21°09′. The bay stretches from Quang Yen town, past Hạ Long city, Cẩm Phả city to Vân Đồn District, bordered on the south and southeast by Lan Ha Bay, on the north by Hạ Long city, and on the west by Bai Tu Long Bay. The bay has a 120-kilometre-long (75 mi) coastline and is approximately 1,553 km2 (600 sq mi) in size with about 2,000 islets. The area designated by UNESCO as the World Natural Heritage Site incorporates 434 km2 (168 sq mi) with 775 islets, of which the core zone is delimited by 69 points: Dau Go island on the west, Ba Ham lake on the south and Cong Tay island on the east. The protected area is from the Cái Dăm petrol store to Quang Hanh ward, Cẩm Phả city and the surrounding zone.
The climate of the bay is tropical, wet, sea islands, with two seasons: hot and moist summer, dry and cold winter. The average temperature is from 15–25 °C (59–77 °F), and annual rainfall is between 2 and 2.2 metres (6.6 and 7.2 ft). Ha Long Bay has the typical diurnal tide system (tide amplitude ranges from 3.5–4 metres or 11–13 feet). The salinity is from 31 to 34.5MT in the dry season and lower in the rainy season.