A small portion of the arterial Zakaria Bunder Road between Cotton Green and Sewri stations on the Harbour line sunk on Tuesday afternoon.
After the civic workers excavated the 4 inches long and 10 inches wide stretch, they found a cavity underneath, which could have developed because of the century-old leaky 48-inch water main underneath.
"The water line had developed cracks and was leaking. After the patch had sunken in we fixed it to avoid a possible cave–in," said a civic official, requesting anonymity.
The portion of the road was barricaded, and traffic diverted. "Work on the repairing the water main has begun. But we will have to excavate further to see if the underground storm water drain line has also been damaged," added the official.
The city has witnessed several incidences of road cave-ins as the underground utilities, have corroded and developed cracks leading to formation of a cavity.
In July, a large portion of Dr Narayan Hardikar road in Worli, which leads towards the Bandra-Worli sea link, caved in.
The road that is perpendicular to the arterial Annie Besant Road gave way after a 90-year-old double barrel arch drain running under the road collapsed.
"These cave-ins can be prevented if the old underground utilities are replaced. The civic body doesn’t even have an underground mapping of all the utilities," said an official from the road department.