Cesspool or Septic Tank?
I'm not sure if every county does this or not, but it is an easy place to start.
If this doesn't pan out, just call a septic service and tell them your dilemma. I'm sure they would come out with a metal rod and poke around to find your tank. It will probably be close to the house where the pipe exits the house (if it's a septic tank).
If you have a septic system you are probably past due to have it pumped.