Quite the mystery is being discovered under the old St.Catharines General Hospital.

A large part of the building came tumbling down yesterday on Queenston Street as demolition crews worked to bring it down.

Mike Starnino, with Starnino Environmental Recovery Inc. tells CKTB the demo is taking a bit longer because they have found tunnels under the hospital.

He says some of the tunnels go to area churches, and others simply take you under Queenston, but he's not sure where they end up.

Starnino says they continue to investigate the tunnels and ensure workers' safety.

Councillor Lori Littleton posted a video on Twitter of the main structure coming down.