It is the Nee Soon Swamp Forest, which was mentioned in this article about Singapore’s very own firefly:
Researchers here have identified a new species of luminous firefly, the first such discovery made in Singapore since 1909.
The Singapore firefly (Luciola singapura), which is less than 5mm long, was discovered in the Nee Soon Swamp Forest, the last remaining freshwater swamp forest in Singapore.
Where exactly is the Nee Soon Swamp Forest?
It is in the Central Catchment Area, bounded by Upper Seletar Reservoir to the north, Upper Peirce Reservoir to the south, and the Seletar Expressway and Old Upper Thomson Road to the east.
The 1923 map below shows the same area at the time.
The swamp forest was bounded by Mandai Road (later realigned because of Upper Seletar Reservoir) to the north, Kalang Reservoir (now Upper and Lower Peirce reservoirs) to the south, and Seletar Road (now Old Upper Thomson Road) to the east. The nearest large settlement was Seletar Village, later Nee Soon Village.
The river in the swamp forest was a tributary of the Sungei Seletar. Perhaps the forest should have been called Seletar Swamp Forest instead.
Instead, the forest probably got its name from the historic Nee Soon area, which was made up of Nee Soon Village (expunged) and Nee Soon Road (still around). However, the Nee Soon area lies outside the forest. The same area in 1954:
Curious toponymics aside, it’s great that the forest lies inside the Central Catchment Area - it should not be threatened in the near future. Long live the forest!