Could it be an old water well? I have a property that came with a concrete
cylinder about 3 feet across and about 8 inches up out of the ground with a
round concrete cap on top. When I opened it up, it was an old shallow water
well and there was a well pump in a crawl space under the house that ran to
the well.

Or, if it is a septic system -- Like you, I don't have any real experience
with septic systems, but I recently found this website that provides a ton
of information about how they are constructed, how they work, etc.:

http://www.septicprotector.com/


Molly Brown wrote:
I saw what looked like a septic tank cover right next to this very
expensive house built on a slab foundation. The house is on a hill and
the ground falls off right after where this cover is so the only place
the septic tank could possibly be, I would assume, is under the house.
Is this possible?