This one is a good one.
I did a bathroom remodel job when I was contracting. It required some new
underfloor plumbing. As I followed the existing waste line under the house
it went underground near the foundation. There was a hole in it and I
discovered standing waste water. I assumed it was a small hole in the pipe
and I would just have the section replaced. I used a small drill pump to
dewater what I assumed was a small volume.
Well 30 min later there was no change in the size of the puddle. I started
to nose around some more. I discovered the pipe terminated in an old steel
septic tank burried about 2' under grade. The homepwner had a new septic
system installed a few years prior and apparently the contractor had just
tied into the outlet from the old tank and went on out to the new tank. Not
only that, the contractor who had added on to the house some years prior to
the installation of the new septic system poured his foundation right over
the top of the old septic tank!!