"Ed" wrote in
:
What can cause this?

Sinkholes?


How can I find out how serious it is?

Call your county extension agency.


If all I have to do is to fill them, what can I use so the grass will
grow again?

My suggestion would be "soil." Just a thought.

If the ground continues to subside, you may have a bigger problem than you
realize. Do you have any mining (open pit or otherwise) going on nearby?
Anything else that could draw down groundwater?