Firecrackers and Chinese Manholes: An Explosive Combo

I’ve been running into these videos where someone in China lights a firework, drops it down a manhole and gets literally explosive results. The ensuing blast is powerful enough to launch the manhole cover!

Is this kind of methane buildup particular to sewers in China, or are there other places where the sewers are brimming with poop gas? Is it simply caused by the high population density in Chinese cities coupled with Chinese cabbage or is there some other cause?