Ha Long Bay

The islands are made of a rock called limestone.

Water can slowly wear limestone away, a tiny bit at a time. Over a very, very long time, water shaped the rock into hills and towers. Then the sea came in and filled the spaces between them.

The islands are made of limestone, and in places the limestone is as much as 1,000 meters thick. Limestone is a rock that water can dissolve. When rain soaks into cracks in limestone, it slowly dissolves the rock, leaving behind holes, caves and steep hills. A landscape shaped this way is called karst.

Ha Long's karst was shaped over about 20 million years in a hot, wet tropical climate, while the land was slowly lifted up. Then the sea flooded in, turning hilltops into islands. The sea level here has risen and fallen many times. Where the waves lapped against the cliffs, they cut grooves called notches into the rock.

The Ha Long Bay Management Board attributes the karst to a combination of thick limestone (as much as 1,000 m), a hot, humid climate, slow tectonic uplift and the full phases of a modern marine cycle. Brooklyn College geologist Guillermo Rocha summarizes the timescale: the limestone has gone through 500 million years of formation in different environments, and the karst itself evolved over about 20 million years under a tropical wet climate. The National Park Service explains the general process: water falling on soluble rock drains into cracks and dissolves the bedrock, creating sinkholes, caves and springs.

The Management Board notes many periods of marine transgression and regression (the sea advancing and retreating). Sea invasions in the late Pleistocene and late Holocene took part directly in the karst process, cutting sea notches and foot caves, and flooding closed, rounded hollows to make saltwater lakes. UNESCO adds that the property displays the full range of karst formation processes, and with Cat Ba it includes every stage of the sea's inundation of tropical karst.

Some islands have caves inside them.

Some caves are very old. Others are right at the edge of the water. Some islands even have a little lake of sea water in the middle!

Ha Long Bay has many caves, and UNESCO sorts them into three main kinds. Some are the remains of caves that formed below the water table. Others, called foot caves, formed at the base of the limestone hills. The third kind are notch caves, cut by the sea at the waterline.

Some islands also hold lakes. These lakes fill hollows in the limestone that the sea has drowned, and some of them seem to rise and fall with the tide. To reach the Cong Do sea lake, visitors pass through a cave at low tide or climb over a 10-meter-high cliff.

UNESCO identifies three main cave types in the property: remnants of phreatic caves (formed below the water table), old karstic foot caves, and marine notch caves. Abundant lakes occupying drowned dolines, closed karst hollows, are a distinctive feature of the fengcong karst, and some appear to be tidal. Locally these enclosed marine lakes are called tùng and áng; the Management Board describes how the sea invaded closed, rounded karst depressions to turn them into saltwater lakes.

The Management Board's Cong Do Lagoon page gives a sense of scale: Ang Cong Do covers 0.5 ha and is reached through Cong Do cave at low tide or over a 10 m cliff, and its floor is sand, gravel and aquatic vegetation. The Board also lists scientifically valuable features such as fossils, folds, fault breccias and sea notches, which it calls evidence of ancient sea levels.

Many plants and animals live in Ha Long Bay.

Forests grow on the rocky islands. Coral and fish live in the sea. A very rare animal called the Cat Ba langur lives on Cat Ba island.

The Ha Long Bay Management Board counts nearly 3,000 kinds of plants and animals in the bay area, including 507 land plants, 110 corals, 156 fish, 71 birds and 53 mammals. Seventeen plants grow naturally nowhere else, such as the Ha Long palm. Caves have their own creatures too, like tiger geckos and the Ha Long cave crab.

The Cat Ba Archipelago is home to the Cat Ba langur, a primate that lives only there and is Critically Endangered. In 2000 there were 47 of them. After 23 years of a conservation project, the city of Hai Phong reported 82 langurs in September 2023. UNESCO also lists threatened animals such as the Asian small-clawed otter, the keeled box turtle and the king cobra.

The Management Board describes ten typical ecosystems of the tropical limestone archipelago, from forest on the islands and cave ecosystems to tùng-áng lagoons, mangroves, seagrass and coral reefs. Its species counts are nearly 3,000, including 507 terrestrial plants, 278 phytoplankton, 141 zooplankton, 110 corals, 156 fish, 71 birds and 53 mammals, with 102 threatened species (64 animals, 38 plants) and 17 endemic plants such as Livistona halongensis. Most species of its cave ecosystem are endemic, the Board says; they include Goniurosaurus tiger geckos, the Ha Long cave crab and the Ha Long cave loach.

UNESCO calls the Cat Ba langur a Critically Endangered endemic primate and the archipelago's flagship species, still affected by disturbance from tourism, restricted gene flow and fragmented subpopulations. Hai Phong's city portal reported that the population had grown from 47 individuals in 2000 to 82 in September 2023, about 3 percent a year on average; the project's 2019-2023 phase was sponsored by Germany's Leipzig Zoo. The Board also warns that seagrass beds have almost disappeared from the bay's coasts because of land reclamation.

Visitors ride on boats to see the islands.

Grown-ups buy tickets at the proper boat port. Keep the bay clean: never throw rubbish in the water. Never break off pieces of the caves. Leave plants, rocks and coral where they are.

The Ha Long Bay Management Board asks visitors to go to the official passenger port to buy tickets and choose a sightseeing route, rather than hiring boats from people outside the port. At the port, an information center can give advice. Once on the bay, follow the rules of each pier, harbor and attraction.

To protect the bay, never throw rubbish or waste into the water, do not break trees or pick flowers and leaves, never break off stalactites in the caves, and do not buy or sell coral. People who look after the bay work hard to keep it clean: in May 2026 one clean-up team collected more than 1,000 plastic cages from the bay's shores and waters on its first day.

The Management Board's visitor notes ask people to buy tickets for set sightseeing routes and overnight stays at the passenger port, seek advice from its information center, and avoid touts and boats hired outside the port. For the environment it forbids throwing garbage into the bay, damaging trees, picking flowers or leaves, breaking stalactites or carvings in caves, and buying or selling coral. On 10 June 2026, before the summer peak, the head of the Management Board inspected lifesaving and rescue operations at Ti Top Beach and asked staff to keep checking lifesaving equipment, weather warning systems and emergency plans for incidents at sea.

UNESCO notes that the property lies in an area of busy tourism, marine transport, fisheries and daily life on the water. It names rising visitor numbers and tourism development as a continuing management challenge, and asks that tourism stay within the property's ecological carrying capacity. The Board reports pressure from tourism, aquaculture and livelihoods at sea; in a clean-up from 23 to 28 May 2026 its Conservation Centre No. 2 removed foam buoys, rafts and plastic cages, and it told owners to remove unauthorized floating structures.