Japanese Tunnel Tai Tam Reservoir [c.1942- ]
Learned about it from a few online sources, also Mr. Cracknells blog as I recall. Went and looked through it this summer. Figured I would add it here. It is reasonably long, going through a ridge I think, 2 exits and a somewhat long side tunnel as I remember. One exit overlooks the reservoir.
The main access point is right off the staircase leading up from the crossroads of Mt. Parker road/Boa vista track. it is only about 50 or so meters up the staircase just past a power station. The entrance is large and obvious.