The hole is near the old Market Square, which is undergoing construction to be the new school of architecture.
Sudbury police were called to the scene around dawn on Monday morning.
Sergeant Corinne Fewster, who reported she could see running water within the hole, said she was able to prevent a van from falling into the cavern before the road was closed off.
"When I first arrived, the far right corner ... had not yet sunk in and I was stopping a van from going," she said.
"I was standing very close to that edge when the far right corner fell in."
The City of Greater Sudbury tweeted early Monday morning that a broken water main was the problem.
City crews are currently working on the fallen section of road.
Police report they do not know when the road will be reopened.