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 have dug down about 4 feet and about 4 feet wide and there I no pipe! I have probed a bit beyond this depth and no indications of a pipe for the ~2 feet 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 any one tell me what this concrete structure with the clean out is for? Thank you. I will keep digging!