The caves were discovered by a team of archeologists from the University of Toronto in 1982. While on an expedition in northern Ontario they found some small painting on a rock wall depicting a cave with people singing in the opening. Following a series of glyphs they quickly found themselves standing in the mouth of a large cave, with smaller cracks and cavities disappearing into the depths.
Standing just inside the threshold they noticed a strange sound as the wind blew through the interior. Almost tonal. Based on the paintings and this sound, they set out to see if this cave had any special acoustic properties.
Gently singing into the opening created a feedback effect, which slowly excited the resonance of the main opening and all of the offshoots deeper within. Finding just the right pitches, the researchers vocalized into the cave until it become so acoustically excited it responded with a loud squelch of reverberation and feedback.
None of the local residents had any knowledge about this place, how it was discovered or made, and what it was used for. Research continues to try and determine if this is natural or created by the people who lived here thousands of years ago.
I didn't include anything about the process for this one, as I kind of like the mystery If anybody's interested after listening to it, I'm happy to share.