What are causing all these Sinkholes?

OPPS first timer! Sorry!
Wanted to get the word out and have an idea and needed some input from all of the brains on ATS!
As you know, man has been taken oil from the Earth now for less then 150yrs.
We fill up the hole with salt water to get most of it out so what does happen after that? Oil by its natural state stays in one place does it not? Now water can seep into the ground right?
So what would be left after the oil is taken and salt water replaced to fill the hole? Has there been any test done to find this out or has time caught up with us?
Any Ideas?

Could it have anything to do with taking water?more and more we're going underground for water, maybe by sucking up underground pockets of water we're hollowing out some areas and encouraging cracks and sinkholes?

because i keep reading that water is getting short on the surface and companies are getting it from under the earth at an increasing rate

just an idea which is kind of opposite to yours, i really don't know one way or the other

It could be water we are taking out of the ground also and with all the dry weather going around is not helping either! Thanks for the imput, God forbid a city falls down into a hole someday.