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.
...so unless this cave happens to be on the way to where you are going, it’s possibly not even worth the walk up from near the waterfalls. It’s slightly worth while, I suppose...at a push, to see the cave that forms the entrance of the mine from which the area draws its name but it’s not much of a sight really.
An interesting geological feature of Tap Mun (Grass Island) is the sea cave located on the western coast near the popular loop trail that circles the southern end of the island. You'll find the sea cave along the boulder beach below the Hilltop Pavilion. It is an interesting place to visit while you are walking the shoreline, around 10-15 minute walk from the Balanced Rock, another notable place to see while visiting the island.
Note: There is an interesting legend regarding Tap Mun that there is a secret tunnel from the Tin Hau Temple that leads to the Tap Mun Cave.