[Cobra64]

Not CO2. Not very effective since we exhale CO2.

CO is deadly..

[Farmer Dean]

CO2 displaces the air,it’s heavier than air.People die in a CO2 atmosphere very quickly.Same with nitrogen.

[Mastador1]

Not CO2. Not very effective since we exhale CO2.

CO is deadly..

C02 is also deadly in large quantities in a confined space, since it displaces breathable air, but it would take a very large amount in a big tunnel. I kind of like a mix of the posted ideas, fill the tunnel with propane and an ignition source while plugging our end with foam and ignite. Timing is important of course, you want as many invaders in the tunnel as possible when you light it up.