Tucked at the bottom of tree-lined slopes along the Kranji Expressway (KJE) is a network of canals that snakes roughly 6.7km from Woodlands Road, through Segar Road, to Bukit Timah Hill. Another network runs 5.8km from Woodlands Road through Bukit Panjang Road to Bukit Timah Hill.

These networks in the northern part of Singapore, and the thick foliage above the 1.7m-deep drains that run next to them, hide a secret - they have been a passage for suspected illegal immigrants who play a game of hide-and-seek with the authorities, and with residents wary of their activities.

Largely men, they try to avoid being seen by keeping their daytime movements to a minimum, and use the canals as roads linking different hideouts that dot the forested areas nearby.

Bukit Panjang residents living near Zhenghua started noticing men behaving suspiciously around the canals last September.

In October, a Straits Times reader contacted the paper hoping to shed light on suspicious activities she noticed close to where she lives near Woodlands Road.

Over the next two months, ST searched for the men who use the travel network, while also alerting the authorities of the suspected illegal activities.

Proof that people had been living in the area was clear - from mattresses to makeshift kitchens. The authorities later removed these items to ensure the men would not return. But the men themselves managed to avoid detection.

All that changed in late October last year - the Bukit Panjang resident who contacted ST pointed out some areas where the men had been seen previously.

One part of the small drain runs less than 60m from Block 636 Senja Road. After a week observing the area, a man was spotted walking inside the drain towards Woodlands Road, but only his head was visible from the vantage point.

In the following weeks, more people were spotted inside the drains, some sloshing in ankle-deep water towards Segar Road. The 1.8km-long trail led to concealed spots on the tree-lined slopes sitting on either side of a drain network.

ST found one "home". Hidden from prying eyes were toothbrushes, soap bars and plastic bags containing detergent and towels. The forest floor was littered with canvas bags, discarded food packets and cigarette carton boxes.

Without warning, two men, who looked to be in their 20s, appeared on a nearby trail carrying plastic bags of groceries. They had come from the nearby Senja Road Housing Board estate.

Realising they were spotted, they quickly vanished near the drain. But, by then, photojournalist Ng Sor Luan had found their "home" on a two-storey-high rocky slope.

Former law enforcement and tobacco industry sources said these hideouts may be connected to others in the vicinity, as the area has a history of illegal cigarette-peddling activities.

"These hideouts are secluded, yet close enough to Housing Board estates where peddlers can buy food and sell their contraband cigarettes," said one former officer.

In October 2018, a raid below the Zhenghua Flyover, about 3.5km away from the makeshift camp, led to the arrest of five immigration offenders from Indonesia and the seizure of a large amount of contraband cigarettes.

Choa Chu Kang resident Leonard Boey is aware that men have been spotted in the forested areas nearby. They would emerge from trails within Dairy Farm, said Mr Boey, who spoke to ST at a jogging trail in Dairy Farm Road.

The 49-year-old said most people are unaware of the dangers of having undocumented people living in the forest or canals.

"I have seen lone women running here at 7pm and teens loitering with expensive camera equipment. People have no idea how remote this place can be," he said.

The canals connect different parts of Bukit Panjang and the neighbouring areas. Our 12km trek above the canals and through the forested areas uncovered more hideouts and makeshift tents.

Near Mandai, an elderly Bukit Panjang resident said he had in recent months seen the police rounding up some men staying in the forest. Bukit Panjang resident Lim Leng Wee, 46, was taken aback when told about the hideouts close to his Segar Road home.

"If they are not doing anything illegal, why do they need to hide?"

PUB launches investigation, removes items

After The Straits Times alerted the authorities to the presence of men walking in drains and living in hideouts near Senja Road, national water agency PUB launched an investigation.

Officials removed several items, including a mattress and personal belongings, found in a drain along Kranji Expressway (KJE) near Woodlands Road.

The uncovered drain, where a man was first spotted in October last year, is about 1.7m deep and 0.75m wide. Running parallel to a slip road off the KJE, it is linked to a covered drain of similar dimensions close to West Spring Secondary School.

It was in the same area that PUB found a metal frame bed and some plastic bags in October 2017.

Between 2017 and last year, PUB had received feedback from a total of four sources on people entering drains without permission.

A PUB spokesman told ST in an e-mail reply: "Members of the public are not allowed to enter drains at all times for their personal safety.

"Heavy rainfall in upstream locations will cause sudden and rapid water flow, posing a serious danger to anyone in the drains."

Anybody who enters the waterways without approval can be fined up to $3,000.

Separately, the Immigration and Checkpoints Authority (ICA) said it has been working closely with enforcement agencies, such as the police, to conduct raids on immigration offenders. In a joint reply to ST, the police and ICA said: "The relevant agencies will also dismantle any makeshift shelters and remove any items found during operations mounted in forested areas.

"Immigration offenders found to be involved in other criminal or illicit activities will be referred by ICA to the relevant agencies for investigation."