COMPLEX CAVES
As the rock here is divided into thousands of islands surrounded by seawater, a large surface area of limestone is exposed to the elements. The sea is constantly eroding the islands and land of Ha
Long Bay, and as a result there’s an abundance of passages and caves. 30 of these caves are currently accessible to people. Ha Long Bay’s cave types include phreatic caves and marine notch caves.
The oldest caves in the bay are the phreatic caves, which formed by carbonic acid from groundwater dissolving the limestone. This created weaknesses in the rock, allowing water to flow in under pressure and creating the gaps in the rock.
Notch caves grow over time at the surface of the sea. As waves continue to crash into the limestone, pieces are eroded and washed out to sea. This process is called abrasion and leaves an indent in the rock at the height of the waves.