Const. Victor Kwong said Monday that the tunnel was found by a Toronto and Region Conservation Authority employee, who then reported it to police.
Kwong would not comment further but said Toronto police have scheduled a news conference for Tuesday morning.
Ontario Provincial Police, meanwhile, said there is currently no threat but they continue to monitor the situation.
A spokesman said Ontario's Ministry of Community Safety is aware of a police investigation "into a tunnel discovered near York University," but could provide no other details.
A Toronto police source said a thorough investigation followed the discovery of the tunnel "about a month ago."
The CBC reported the tunnel was about 2.5 metres deep and about seven metres long before it was filled in by authorities.
The broadcaster said the tunnel had lights inside and was powered by a generator.