Along the coastlines of Hong Kong, you can find lots of sea caves, which are formed primarily by the wave action of the sea at the weak zone of the rocks.
Mok Min Cave (木棉桐) is the best-known sea cave that can be reached on land. When the tide is low, people can get inside and walk through.
Mok Min Cave is close to MacLehose Trail Stage 1 (40 mins walk), but there is no road sign on the way. Make sure you check the map before you go there.