In message , FMurtz
writes

Why not ask the builder or owner?

I did :-) Caught a guy yesterday, and I was wrong. Whatever is
underground is nothing to do with soakaways or septic tanks, and is part
of the heating system, but that is as much as I know. All that can be
seen now is what looks like a couple of plastic manhole covers above
ground.
--
Graeme