Beneath the well known Peak of Mount Faber, Singapore lies a relatively unknown subterranean complex: Faber Fire Command Fortress Plotting Room, which was built in 1936.
The Fortress Plotting Room was where information about naval targets was received from Fortress Observation Posts, plotted and the required information extracted and then acted on. There were two Fortress Plotting Rooms in Singapore. The other was Changi Fire Command Fortress Plotting Room. In addition to the Fortress Plotting Rooms, Counter Bombardment gun batteries each had a Battery Plotting Room, which received information from the relevant Fortress Plotting Room and their respective Battery Observation Posts. 
A long tunnel leads into the complex from the North side of Mount Faber running beneath Mount Faber Road. The rusty metal gate on the tunnel lies corroded in the overgrowth having decayed over many decades. (Note: A new secured gate and lock has been installed as of 08/2019)
The concrete lined access passageway extends a fair distance into the hill, with just about enough headroom to walk. Due to its small size, my thoughts are that this passageway was a secondary or emergency exit, as opposed to the main entrance from the South Side of Mt Faber.
The passageway then emerges into the main chamber of the underground complex, a much loftier space with a corrugated metal ceiling.
An open door leads to one of the Fortress Plotting Rooms which previously contained a Range-finding Exchange (telephone), Fortress Plotters to track shipping targets on charts, Fall-of-Shot Indicators, Fire Direction Table, Ballistic Correction Calculators and other equipment to collate incoming information about the type of target and its range, speed and course. 
Mineral deposits hang from the ceiling, with signs of water seepage.
Moving back to the main space, a long flight of stairs leads up to the main entrance which arises on the South side of Mt Faber, near the Loop road. The revelation here being that the complex actually cuts through the hill itself – a 30 second walk between both sides here would take at least 15 minutes by foot above ground!