[pierrem15]

That's a good guess. There are some lakes in Africa that are so charged with CO2 that sometimes they boil over and the gas slides silently downhill and suffocates an entire village.

If there were a pond or lake emitting CO2 or some other non-stinking inert gas it could displace the air and they'd have just walked into it and a couple minutes later collapsed, just like an enclosed space death.

[Lonesome in Massachussets]

Death by carbon monoxide is insidious and painless. Death by carbon dioxide is painful and excruciating. Our body is finely tuned to detect and expell carbon dioxide. Respiratory distress is caused by inability to rid the bloodstream of carbon dioxide.

[arkfreepdom]

I forget the name of the algae but it emits a deadly gas.