This is kind of a wacky theory I have. I remember reading about a brewery that was so successful it used up so much ground water causing the city to sink. They had to close. Well, if there is so much oil being pumped out of the earth couldn't this cause air pockets causing masses of earth to sink? I mean think of the pressure under the oceans, then you have huge cavities. It seems that some day they would collapse. Possible?