Subject: carbonic púddels and caves

the other day i saw on tv that in pre-history someitmes a "puddle" of carbonic
gas (I alwas mix dioxide with monoxide) would form in some places, and when soem
animal entered it it would die from the lack of oxygen (or poisned by the gas, I
dunno), my quesiton is, if those kind of gasses fall down, why aren't the lower
parts of caves lakcing in breathable air?

Re: carbonic púddels and caves