Oil.

Two random observations:

1. I'm under the impression that oil is used for a lot of very specific things, among them making plastic. It's a really neat substance. I don't know how much of the "stuff" we have today in which oil is a necessary ingredient, but I'm guessing it's a whole lot.

2. Also, oil is probably one of the most rare substances in the universe, based on the fact that it's what happens to organic lifeforms after millions of years. We don't know any other planet on which it exists.

And we're burning this stuff?