I read about this tunnel recently in an 1986 newsletter "Military History Notes", written by Philip Bruce:
Perhaps the most extensive Japanese system to be found in Hong Kong is on Smugglers' Ridge, to the south of and overlooking the Shing mun redoubt. There are at least two entrances on the northern face, about 30 feet or so down from the ridge path, well concealed by undergrowth and requiring some effort to locate. It is worth noting that they were uncovered only recently by a former US Marine officer with extensive Vietnam experience. <Read more ...>
The main entrance is a drop into an a shallow pit. Behind it they'd excavated a cavern into the rock, and on the left side there is a tunnel. The tunnel leads down to a smaller opening a short distance from the main one.
