The Merdeka Bridge near the Golden Mile Complex hardly attracts a second glance from passers-by.
But a section under the bridge, which can be accessed only via two openings just big enough to crawl through, was suspected to have been a drug den.
Last week, two Thai nationals were nabbed there for drug-related offences.
Work-permit holder Srichan Cha Kupong, 30, was also arrested on suspicion of possessing an offensive weapon.
His female companion Bunkoy Rungtawan, 34, who is on a social visit pass, is suspected of immigration offences.
Both have since been charged.
It is unclear how long they had been using the area below the bridge.
The Sunday Times investigated the area before the arrests.
There were several signs of habitation.
Mattresses, clothes lines, shoes, toiletries and even luggage were found tucked in a corner.
The low ceiling made it hard for anyone taller than 1.6m to stand upright.
The air reeked of stale food and urine.
Empty beer cans and plastic bottles littered the ground.
The only light seeping into the hideout was from the crevices at the sides of the walls.
Packets of stale rice and rotten bananas in plastic bags hung from metal pipes running below the ceiling, away from ants and cockroaches scuttling on the dirt ground.
A series of four-digit numbers and Thai words were written in red marker ink on concrete walls.
The noise of the traffic above was absorbed by the concrete slabs and reduced to a faint hum.
A middle-aged man in a long-sleeved black shirt and trousers was lying on a mattress.
When asked if he lived there, he replied in Mandarin: "Other people also come here."
He walked away briskly with his backpack.
A 43-year-old Singaporean volunteer with a non-governmental organisation (NGO) said he visited the hideout three weeks ago.
A park user had alerted the NGO to the people seen going in and out of the area under the bridge on Sundays and public holidays.
"The place was very dirty and I could smell urine," said the full-time photographer who declined to be named.
"I wanted to explore the place further but I quickly left when I heard a man and woman speaking in Thai."
A spokesman for the Ministry of Social and Family Development said its staff have recently conducted checks on the area and found "possible signs of illegal activities".
The matter has been referred to the police.
The police said they are working with the Land Transport Authority to seal the entrances to the area, adding that it will continue to monitor the situation closely.
The National Environment Agency has since cleaned up the place.
Additional reporting by Hoe Pei Shan and Amos Lee
