Ha Long Bay (Vịnh Hạ Long, “descending dragon bay”) is a bay of about 1,553 km² on the coast of Quảng Ninh in north-eastern Vietnam, filled with 1,969 islets, most of them steep limestone towers covered in jungle and some hollowed by caves. It formed when the sea rose over a landscape of karst hills after the last Ice Age, drowning the valleys and leaving only the summits above water. UNESCO listed the bay as a World Heritage Site in 1994 for its scenery, added its geology in 2000, and in 2023 extended the site south-west to take in the Cát Bà Archipelago. This guide explains how the bay was made, then follows the route of a boat trip through its caves, floating villages and islands.
500 Million Years of Limestone in Quảng Ninh
The rock of Ha Long Bay is old. Its limestone has formed over about 500 million years in changing seas and environments, in sediments that hold fossils of corals, bivalves, brachiopods and foraminifera, and the region’s rock record runs from the Precambrian to the present, with ten breaks in the sequence. The karst itself, the landscape of towers, sinkholes and caves that forms when rainwater slowly dissolves limestone, is far younger. It has been developing since the Miocene, over roughly 20 million years of hot, wet tropical climate and slow uplift.
That combination is what karst needs: a great thickness of strong, pure limestone; heavy rain, which here totals 2 to 2.2 metres a year; and slow tectonic uplift to keep exposing new rock. Water ran down joints in the rock, widened them into sinkholes, or dolines, and then into gorges. Over time the land between the hollows became separate hills. Average temperatures in the bay today range from 15 to 25 °C, with a hot, humid summer and a dry, cool winter.
Drowned Towers: Fengcong, Fenglin and the Rising Sea
Geologists use two Chinese terms for the shapes that resulted. Fengcong is karst of clustered cone-shaped hills joined at the base, which you can still see on Bồ Hòn and Đầu Bê islands, where the cones average about 100 m high and the tallest pass 200 m. Fenglin is karst of separate towers rising from a flat plain. Most of the bay’s famous islands are fenglin towers, between 50 and 100 m tall and up to about six times as high as they are wide.
Before the sea arrived, those towers stood on a coastal plain. The bay in its present form appeared after the sea reached its highest level in the middle of the Holocene, a few thousand years ago, flooding the plain and the sinkholes alike. The sea then went to work on the rock. Seawater dissolves limestone quickly, and waves cut a deep notch at the foot of the cliffs; in places the notch has been enlarged into a cave. The bay has one tide a day, with a range of 3.5 to 4 m, so the notches are alternately flooded and exposed.
The drowned sinkholes produced something stranger: lakes inside islands. Đầu Bê island, at the mouth of the bay, has six enclosed lakes, including the Ba Hầm group, reached through a curving tunnel about 150 m long and 10 m wide at water level. Shells of oysters stuck to the old undercut notches in the cliffs have been carbon-dated to between about 2,280 and more than 40,000 years old, recording earlier sea levels.
Island counts vary because so many are tiny rocks. The figure of 1,969 islets covers the whole bay; descriptions of the classic seascape often say “some 1,600” limestone islands; and 989 of them have names. Many are named for their shapes, such as Voi (elephant), Gà Chọi (fighting cock), Khỉ (monkey) and Mái Nhà (roof). The World Heritage zone in the centre of the bay covers 434 km² and holds 775 islets.
On the Water: Sửng Sốt, Thiên Cung and Đầu Gỗ Caves
Most visitors see all this from a boat. The caves they stop at are not the sea-level notches but much older chambers left high in the islands, remnants of passages dissolved by groundwater when the karst was still forming on dry land. Sửng Sốt, the “Surprise Cave”, is on Bồ Hòn Island. Its entrance chambers sit on a ledge high on the cliff, and from there a passage more than 10 m high and wide descends to the south among stalactites and stalagmites.
Thiên Cung (“Heavenly Palace”) and Đầu Gỗ are two surviving parts of one old cave system in the north of Đầu Gỗ Island, between 20 and 50 m above sea level. Thiên Cung is a single chamber more than 100 m long, almost divided into smaller rooms by a wall of stalactites and stalagmites; local legend makes it the scene of the Dragon King’s seven-day wedding. Đầu Gỗ, the largest grotto in the Ha Long area, is one long tunnel descending along a set of fractures. French visitors in the late 19th century called it the Grotte des Merveilles and left graffiti on its walls.
Its Vietnamese name, “cave of the wooden stakes”, ties it to the bay’s best-known battle. In 1288, on the nearby Bạch Đằng River, General Trần Hưng Đạo planted steel-tipped wooden stakes in the riverbed at high tide; when the tide fell, the stakes sank Kublai Khan’s Mongol fleet. Local tradition holds that the wooden bars found in this cave are remnants of those stakes. Smaller stops include Trinh Nữ, an old foot cave about 80 m long with its ceiling some 12 m above the sea, and Luồn Cave on Bồ Hòn Island, a low water-level tunnel that runs about 50 m into an enclosed tidal lake.
Cửa Vạn and the Floating Villages
Only about 40 of the islands are inhabited, and the bay’s best-known residents live on the water itself. Around 1,600 people lived in four fishing villages, Cửa Vạn, Ba Hang, Cống Tàu and Vông Viêng, in houses and rafts floated on tyres and plastic drums. They fished and farmed the shallow waters, which hold about 200 species of fish and 450 kinds of molluscs, and in recent decades many added homestays, boat rides and seafood meals for tourists.
That way of life is shrinking. To protect the water and the landscape, the Quảng Ninh provincial government has been moving families ashore; since May 2014 more than 300 households have been resettled at Khe Cá, on the mainland, and the province plans to keep only a few villages as places for visitors to see. People have lived around the bay for far longer than the villages suggest. Archaeologists recognise a sequence of local cultures, from the Soi Nhụ, which began around 18,000 BC, through the Cái Bèo to the Hạ Long culture of 5,000 to 3,500 years ago.
From 1994 to 2023: UNESCO and the Cát Bà Extension
The bay was praised in poetry long before it was protected. Around 500 years ago the scholar Nguyễn Trãi called it “a rock wonder in the sky” in his poem Lộ nhập Vân Đồn. Older Vietnamese texts used other names for it, such as An Bang, Lục Thủy and Vân Đồn; the name Hạ Long first appeared on French maritime charts in the late 19th century, and the dragon legend behind it was reinforced by 19th-century Nguyễn dynasty nationalism. On 5 June 1948 the bay also gave its name to the Hạ Long Bay Agreements between France and a provisional Vietnamese government.
In 1962 North Vietnam’s culture ministry designated the bay a Renowned National Landscape Monument. UNESCO inscribed its core in 1994 under criterion (vii), for exceptional natural beauty, and in 2000 added criterion (viii), for its record of the Earth’s history and its karst landforms. In 2023 the property was enlarged and renamed Hạ Long Bay – Cát Bà Archipelago. The Cát Bà Archipelago, administered by Haiphong, consists of 367 islands covering 262 km² and has been a UNESCO Biosphere Reserve since 2004. Its main island holds Cát Bà National Park, created in 1986, and the Cát Bà langur, a golden-headed monkey found nowhere else, with fewer than 70 left in the wild.
Coal, Ports and Plastic: Keeping Ha Long Bay Clear
Fame has brought pressure. The coasts of Quảng Ninh and Haiphong have grown fast through shipping, coal mining and tourism, and the Asian Development Bank warned as early as 1999 that new ports would increase traffic in the bay. Mangroves and seagrass beds have been cleared for jetties, coral reefs have declined, and in 2016 stalactites were reported cut from caves for decoration. In October 2011 the World Monuments Fund put the bay on its 2012 Watch list because of tourism-related development.
The province has responded by banning coal and stone extraction within the heritage area and prohibiting high-speed tourist motorboats, and from 1 September 2019 Ha Long City banned single-use plastic in the bay. In 2012 the New 7 Wonders Foundation also named Ha Long Bay one of the New 7 Wonders of Nature. To plan a visit around the weather, read our guide to the best time to visit Ha Long Bay; for the science of the towers, see what karst topography is; and for the wider country, our Vietnam country guide has the essentials.