A top Jelutong Tower, in the middle of the Central Catchment Nature Reserve (CCNR), we took a deep breath and gazed upon the stunning view.
There were acres of green all around. Birds darted in and out of our view.
In the distance, vestiges of the concrete jungle that is Singapore, like the office buildings along Thomson Road and Housing Board flats in Toa Payoh, can still be seen - a stark reminder of how we were in a pocket of green surrounded by grey.
Biologists worry for this patch with 413 species of plants, 218 species of birds, 30 species of mammals, 24 species of freshwater fish and 17 species of amphibians - some of which are endangered.
They say the work to construct the Cross Island Line (CRL) could lead to water pollution, soil compaction and plant damage - all of which would be irreversible.
Even before the actual excavation happens, the boreholes that have to be drilled to investigate the soil will already damage the environment, they maintain. Of the 37 boreholes, 16 are within the nature reserve.
But in a report published in February, the Land Transport Authority (LTA) said these soil investigations will have a "moderate impact" on the nature reserve.
Plant expert Lahiru Wijedasa, a former senior botanist at the Singapore Botanic Gardens, is not convinced.
He says the boreholes - which are vertical shafts in the ground - will need to be dug by heavy machinery.
"The weight of the 2.5 tonne machines would not only damage tree roots and kill them, it could also cause compaction of soil, preventing anything from growing after."
He says this is alarming because a portion of the nature reserve is primary rainforests - untouched parts of nature which are home to endangered species like the Greater Slow Loris and the Sunda Pangolin.
Biologist David Tan from the National University of Singapore, who has been studying the forest for more than four years, says the loud noise from the digging would also scare away animals, leading to territorial conflicts.
But the biggest concern is water pollution.
Each borehole, says Mr Wijedasa, is going to need at least 1,000 litres of water, which goes into the soil when digging.
"When you pump in water underground like that, based on the method proposed, sludge will be pumped out. If this sludge gets into streams, it will pollute the water which the animals and plants all need to survive," he stresses.
In its report, the LTA has detailed mitigation measures, which include having 30m buffer zones around streams and marshes, and using enclosures to minimise noise and tanks to hold discharge, such as drilling fluid.
Nature lovers like Mr Tan and Mr Wijedasa are urging authorities to consider alternative routes.
LTA has said a route which skirts around the CCNR will cost an additional $2 billion, and require the construction of longer tunnels and more underground ventilation facilities.
Mr Tan feels there is no price too big for conservation.
He, Mr Wijedasa and other volunteers have been leading tours around the CCNR under the Love Our MacRitchie Forest campaign, which hopes to raise awareness about how special the CCNR is.
They hope that through public education, more people will speak up for the CCNR and an alternative route will be chosen.
Mr Tan says: "We should really think about what we value because conservation is something that benefits everyone.
"MacRitchie is not only the biggest forest we have, it is one of the few forests we have left."
What Animals Are There
The Central Catchment Nature Reserve is home to 413 species of plants, 218 species of birds, 30 species of mammals, 24 species of freshwater fish and 17 species of amphibians. They include:
They are a common sight along the trails of MacRitchie and have a diet of fruits, leaves and small animals.
GREATER SLOW LORIS
Their numbers have been dropping due to illegal pet trade and habitat destruction. The species is classified as "vulnerable", which means it is likely to become endangered.
BLACK-BEARDED FLYING DRAGON
This lizard is capable of expanding its ribs out to form "wings" to glide between trees.
GIANT FOREST ANT
Despite being one of the largest ants in the world, this insect is completely harmless.
This animal is critically endangered and is one of the most heavily poached animals in the world. In February, a baby pangolin was found in Upper Thomson Road. It is now thriving under the care of vets at the Night Safari.
This shy mammal moves through the forest quickly due to its small stature. They feed on fallen fruits, shoots, young leaves and fungi.
Additional reporting by Ng Jun Sen and Ariffin Jamar
This article was first published on April 10, 2016.
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