I inherited an old house and am trying to fine the septic tank. There are no records at the town hall or health department, and no one is left alive who knows where it is. I have probed the ground and looked for low, greener or other signs and can't find it. So I dug down just outside the foundation from where the main sewer pipe stack goes into the basement floor. I dug down about 4 feet and about 4 feet wide and there is no pipe! I have probed a bit beyond this depth sideways and got no indications of a pipe for the ~2 feet down I can probe. There is a relatively small concrete "box like" area where the pipe goes into the floor with a cleanout as shown in the attached picture. Can anyone tell me what this concrete structure with the clean out is for? Thank you. I will keep digging!

I don't see any picture or a link to a picture currently.

you have someone send a special cable into the pipe that can be located and followed from above ground outside.

I agree with Packy. This process is well established in the plumbing network. Leave this job to the professional. Its an artform.

Best Wishes