translation.en: And of course, coalmines are very dangerous places because in some of these coalmines, you get gas. When that gas blows up, people die. So you're producing a biogas out of coal in some mines, but not in others.
translation.zh: 当然，开采煤炭是非常危险的事情， 因为在一些煤矿里，你会 发现有毒气体。当这些气体爆炸时，是会死人的。 这些生物气体（比如沼气）存在于有些 煤矿，另外一些煤矿则没有。