Anti-Fizz

Ross pulled this bottle of tonic water out of the cupboard. Tonic water is normally like soda pop: the water is carbonated and the bottle is pressurized. This bottle is several years old, and not only is the fizz long gone, but it evidently continued to pump gas out of the bottle after the pressure was all gone. If I had left it be would it eventually have pumped all the water out too? I suspect some kind of chemical reaction, but I don't know what it would be. This is just all around too weird.

Update February 2017 replaced missing picture.