Ha Long Bay has many caves on different islands, one of the biggest cave systems is Hang Sung Sot on Bo Hon island..
Hang Sung Sot cave on Bo Hon island was discovered by French explorers in 1901 and they named it “Grotte des surprises-the surprise cave”sin French. The name persists, but now in Vietnamese. The cave system has two chambers and it is outfitted with easy to walk trails and steps, so it is mostly easy walking after you climb up the stairs from the pier to reach the entrance to the caves.
The caves are full of both stalactites (extends from the cave ceiling downward) and stalagmites (from the bottom up). There are also formations called flowstone which is a more sheet like formation of calcite and other minerals left when water flows down the walls or on he floor of the cave.