Tai Tam Tunnel [1887- ]
The Tai Tam Tunnel is one part of the Conduit from Tai Tam Reservoir to Albany [Filter Beds] [1887- ]. For more information about this early tap water project see here.
Tunnel works started in 1883 from both sides (Tai Tam and Wong Nai Chung), the technique applied was drill & blast (dynamite). Hard granite made the works pretty tough. Both sections met in mid-1887. The tunnel is 2,2 km long with a diameter of 1,5 m, it is unlined. [Source]
The inlet at Tai Tam is beneath the Recorder House. The location of the outlet on the northern side is unclear (to me). The 1885 map on hkmaps.hk shows it east of Wong Nai Chung Road, possibly near todays Marymound Secondary School. This implies that the conduit started further east and possibly ran along todays Blue Pool Road (sic!). The course of the tunnel was taken from the 1895 map at hkmaps.hk.
Update: the tunnel outlet is near the bend of Tai Hang Road, a bit further north than estimated earlier.
The tunnel likely still exists and is still in use.