This question has actually been asked but my scenario involves different circumstances. I have a can of Coke from 2016, that I was keeping because it was a promotional one. Once, after a about a year past its best before date it swelled up considerably around the base. Anyway, I completely forgot about it and moved one with my life. I found it today in the attic and to my surprise it was almost completely empty. It also wasn’t swollen anymore and the bulge at the bottom of the can had now become concave again although I did have a few cans at the time so I can’t be 100% sure it is the same can. What is intriguing the most is that when I shake the can the remaining liquid still fizzes, so if it is in the process of the evaporating through a pinprick hole how is it still carbonated? Can anyone shed any light?
It might be a 'pin prick' hole towards the base of the can. That hole was significant when it was in bulged form and leaked significant amont of liquid slowly and that leaked liquid got evaporated without leaving any trace on the can. Since the hole was on the liquid side hence it does not leak any co2. Over time, as more and more liquid got leaked, internal preassure got reduced, some of the dissolved co2 also got out from the liquid into the empty space inside but not all, bulging started to go away and the pin prick hole also started to reduce. Due to the less liquid coupled with less pressure, the sugar might got oversaturated and started to crystalize. Few of these sugar crystal might have clogged the pin prick hole from any further leaking of liquid. This is the situation your coke can might be in now. Since the internal pressure of the can was never close to the outside air pressure, hence there are still some co2 left in the luquid in disslived state, that is still causing some fizzle.