"Tom Quackenbush"

| I should also mention that CO is heavier than O2 so the atmosphere
| at the bottom of the rat hole will be have increasingly concentrated CO
| levels. If all else fails the critter will have one h*ll of a head ache.
|
| C = 12, O = 16, N = 14
|
| CO = 28, O2 = 32, N2 = 28

The real world physics/dynamics is not quite that simple but sufficient to
say CO is heavier than air and will settle to the lowest level i.e. the
bottom of the rat hole.