CO2 heavier than air - but it is lighter than rock, so if you put it underground it will tend to come up to the surface, unless there is a physical barrier or chemical 'locking in'.
I think mmgw is happening, but I have no idea what the best thing to do about it is, short of stopping burning fossil fuels, which aint going to happen, and still wouldnt be enough. This kind of thing is a useless and expensive waste of money feelgood sticking plaster.