Islands and Karsts

The most outstanding feature of Halong Bay and undoubtedly its greatest attraction are the 1,696 islands, islets and rocky karst outcrops. 989 of these islets and rocky outcrops have been been given imaginative names by the locals over the years. These have been derived from their perceived shapes and include names such as Man’s Head, Fighting Cocks, Wading Ox and Wallowing Buffalo.
 
With heights ranging from 50-100 meters (160-330 feet) and a height-to-width ratio of about 6:1, the majority of these karst pinnacles and islets have sheer, vertical cliffs. They are uninhabited and unaffected by human activities. The pure size and numbers of these outcrops provide a stunning contrast and backdrop to the turquoise waters of the bay when viewed from sea level.

Caves and Grottos

Another exceptional geological feature of Halong Bay is the number of limestone islands which have caves and grottoes within them. The total number of caves has never been determined and there are almost certainly more which are yet to be discovered. In order to preserve the natural state and integrity of the large number of caves and grottos in the World Heritage Area, only some are open to visitors. The most commonly visited are Thien Cung (Heavenly Palace) cave, Dau Go (Wood Tip) grotto, Sung Sot (Surprise) cave and Trinh Nu (Virgin) grotto.
 
The shapes and formations on the sides and rooves of the caves, many now being part of local folklore and legends, bear witness to the different stages of formation over the ages. Some cave features, such as calcified shell-beds, are the direct result of ancient human activity.

Sea and Lakes

The lower chambers of some caves and grottos are below sea level. Due to the porous nature of limestone, these chambers are often partially filled to form tidal lakes and pools. Although most of these are shallow, the depths of some have never been determined. The geomorphology of Halong Bay indicates that it is almost certain that there are undiscovered and fully entombed lakes within many of the islands. Dau Be (Goat’s Head island) alone has 6 different tidal lakes.
 
The sea in Halong Bay is generally less than 10 meters (33 feet) deep and boasts a rich biodiversity of about 1,000 species of marine animals. Over 160 species of coral have been identified. Coral reefs make up 30% of the seabed and in some areas as much as 80% of the seabed is covered in coral reefs. Most coral reefs are at a depth of 4-6 meters.