He was curious when he saw a man emerge from a drainage canal under a foot bridge at Bukit Panjang Road a few weeks ago.
The 43-year-old IT engineer, who wanted to be known only by his first name, Leon, decided to discover the truth for himself last Tuesday.
Armed with a torchlight and dressed in bermudas and slippers, Leon entered the canal on his own.
He was shocked to discover it to be the secret home to an unknown number of people.
As he wandered through a 600m long and 2.5m deep section of the canal, he came across a fully functioning home, which had two sleeping areas, a bathing area and even an area to wash clothes.
Leon saw two giant beds - one holding three mattresses and the other, two - in two locations along the canal. They had been erected on bamboo stilts to a height of about 2m to protect the dwellers from rising water.
"No other residents know about this place," said Leon, who lives nearby at Bangkit Road.
Most of the secret hideout was in a pitch-black tunnel and an open area shielded from prying eyes by thick vegetation.
Yesterday evening, Leon took The New Paper into the underground system, which can be accessed, perilously, only from a tributary drain.
None of the mysterious dwellers were there at the time. Walking nearly the entire stretch in ankle-deep water and guided only by Leon's torchlight, we got an idea of who might be living there.
These possessions were kept dry above the murky water by being hung on bamboo poles jammed into cracks and holes in the walls.
Four toothbrushes, which looked clean and recently used, were stuck into a crevice near a broken bathroom mirror on the wall.
FAMILY LIVING THERE?
Items like a child's school bag and pairs of women's shoes suggested that a family could be living there. A photograph of a woman was stuck on another mirror.
A guitar, a football, and some plastic toy balls were seen on the "clothing racks".
It was Leon's fifth time there, and he said he had previously seen used condoms in the canal.
When we were heading out to the entrance, we saw a man dressed only in shorts walking in the drain towards us carrying large bags.
As soon as he spotted the lights from our torches and mobile phones, he dropped the bags and bolted in the other direction.
Inside the bags were about 20 empty 1.5 litre bottles, which he was presumably going to fill from a pail near us. The pail had collected water flowing out from a hole in the wall.