one of two tunnel systems I found skirting round Sai Wan Fort. This one has two entrances, one overlooking what was the bay of Sai Wan. Fairly extensive. The main entrance is easy to find (once you get there): on the road up to the fort, head north around the hill from where there is some construction next to a resting area. There is a few minutes of bush whacking but there is a path created by workers who've been making the hillside safer. <Read more ...>
Mr Mr Au Chi Ho, The author of a new book 'Up Where We Belong' mentioned this place. He and his team took quite a load of recent aerial photos. This location was mentioned in page 128. It is only printed in Chinese so far. ISBN 978962144961, selling at HKD138.- Some credit card discount might be application in some book stores if your bill is over HKD200.-
From the shape of the ruin it looked like one of those pre-WWII observation posts with Lyon lights to me.