Authorities have yet to figure out the cause of a fire that destroyed an abandoned building in Gravenhurst. It happened near the John St. dog park around 11:30 Thursday night, with around 30 firefighters from the Gravenhurst Fire Service tackling a fully engulfed derelict building. It took a few hours to knock it down too. We received a note from Gravenhurst Fire, they want to thank the neighbours who brought out refreshments as they worked on the fire. No injuries were reported.