If methane is lighter than air, what killed all those people in the methane pit? I thought it was that methane was HEAVIER than air, which is why it was filling the pit.

Yet the chemistry I remember does say that CH4 should be plenty light... lessee, H=1, C=12, so 1 molecule of CH4 = 12+4=16. On the other hand, 02 = 2*8 = 16 also, so now where are we? And NH3 would be 7+3=10 which is lighter than either CH4 or O2...