Power company SP Group discovered two ancient fault lines while excavating a labyrinth of electricity supply tunnels 60m underground.
The fault lines - in Yishun and Jurong - pose no danger, however, as Singapore is now not in a seismically active zone.
The faults themselves were created about 200 million years ago.
They join at least nine other fissures that geologists know to run deep beneath our feet.
SP Group geologist Tin Moe Moe Naing said the discovery provides a glimpse into the ancient processes that created today's Singapore - processes that were covered by earth and rock over millions of years. "Only by deep excavation and tunnelling will we have chance to find these features hidden underground," said Ms Tin.
The firm found the fault lines between 50m and 75m underground while digging the $2.4 billion tunnels that would house 1,200km of extra-high-voltage cables, a project that began in 2012.
The Jurong faults were discovered between 2014 and 2015, while the Yishun faults were found in early 2016.
Another SP Group geologist, Mr Bryan Kyaw Thet Oo, said the team realised they hit the fault lines when the rock they were excavating became extremely crushed, with a high sand content.
The tunnels did not have to be rerouted, but work had to proceed more carefully through the area.
This is not the only new evidence of past cataclysmic events to have hit the area.
Mr Kyaw said tunnelling around One-North unearthed a layer of highly compressed volcanic ash, a type of rock known as "metatuff".
He said the rock in which this was found is about 200 million to 250 million years old.
"One of the ways it could have formed is through a huge volcanic cloud that settled here," he said.
Research fellow Wang Yu from the Earth Observatory of Singapore noted: "As these old fault lines are deep underground, they do not affect the Singaporean's everyday life.
"However, knowing these fault lines is very important for the construction industry, especially for the development of underground tunnels and caverns.
"This is because the fault zone could indicate an area where the rocks are weak, creating engineering difficulties during construction if they were not identified properly beforehand."
He added that the fault lines could even help researchers to locate groundwater.
Referring to the metatuff find, Dr Wang said volcanic eruptions in nearby countries like the Philippines and Indonesia have affected Singapore throughout its history.
"For example, during the 1991 eruption of Mount Pinatubo in the Philippines, more than 2,000km away, winds blew ash all the way to Singapore," he added.