FWIW -I'm just re-reading the links provided to me above .. and I thought I'd mention this..
you know how they keep mentioning that besides searching the house, the underground maze of rooms, and the septic tank, they did some excavation in the back yard?
We have had trouble with our septic for a few years now and we've had to have my husband's friend come in with a backhoe several times to dig up the yard so he could access the outlet pipe and remove the boxer shorts that were clogging it that first time for example, and also to use an auger to ream it out, for lack of a better term. So I am fairly certain the excavation woudl be related to the septic/clogged pipe JMO and FWIW.
I knew nothing about septic systems till we move out here, we always had city water and sewer before. It's startling how much of a learning curve there really is IMO.. from septic safe toilet paper to not flushing tampons or condoms (no that's not TMI, the fact that I've had my tubes tied might be though - point being that's not something we've had issues with but we were indeed told that's another thing to NOT put in a septic system), to having to use bleach twice a year throughout the system to keep it safe for use, etc etc... so thought I'd mention it. You have to get the septic tank emptied every couple of years regardless. I wonder who they usually use? And if they ever found anything suspect in the past?