One of the interesting geological features to see along the UNESCO GeoPark family trail at the High Island Reservoir East Dam is the partially formed sea cave located in the back of the small buffer lake in between the sea and the reservoir.
You will first enjoy some incredible volcanic hexagonal columns while walking down from the main dam into the lower dam and buffer lake area. Then, following the small road to the back end of the buffer lake, there is a wood platform deck that extends into the lake stopping just in front of the partially formed sea cave, which you can view from a fairly close distance.
The sea cave is the result of thousands of years of slow sea erosion that cuts surface rock away from the main rock face. In time, erosion results in a small cave formation. Eventually, once the eroded area becomes too large and the overhanding ridge above too brittle, it will collapse, destroying the cave and creating a new and separate island off of the mainland. This is what you will find at the nearby Po Pin Chau, around 20 minute hike to the south. It is nice to view these naturally land formations one after each other if you have interest in geology and plan to visit this area of the Sai Kung East Country Park.