Hope this isn't too OT but I thought someone here might have an idea for
me. I'm currently renovating a 100+ year old brick building. Immediately
behind the building, I'd noticed some subsidence in roughly an 'L' shape
when I bought the property. Yesterday, I finally got around to taking a
shovel to the loose the sinkhole that had been forming (over years) in the
lower leg of the 'L'.

In the ground (subsurface) is a brick retaining wall along one side of
the lower leg of the 'L' with two sewer pipes sticking through it. The
pipes are about 12-16" below ground level and appear to have originally
terminated there (inside the 'L'). We've dug five feet down and have yet
to reach the bottom of the retaining wall.

My initial thought was that this was a sewage pit that emptied into one of
the many caves or underground streams that exist in the area. This
occurred to me as a bit fanciful and doesn't necessarily explain the 'L'
shape. Someone else however, has suggested that it could have been
an old backfilled septic tank. The latter idea seems a bit peculiar,
though. A septic from the early 1890's, made of brick with no means by
which to drain? City sewer has been available here since the dawn of time
and the functioning sewage pipe for this building lives 6-8' below ground.
To my knowledge, there wouldn't have been a method to empty a septic
'tank' in the 1890's so there would most certainly have been capacity
issues if this were true. This is not to mention the stench it would have
generated, esp. given it's proximity to the building.

Any other ideas? I checked an old map (1948) at City Hall and there's no
record of another (above-ground) structure having ever been in this spot.
The lower leg of the 'L' is about 6' long x 4' wide and the back of the
'L' is approx. 9' x 4'.

It's shape, size and the construction of the aforementioned wall
effectively rule out any prospect of it being the remains of an outhouse.

I'm stumped and the backhoe doesn't get here until next weekend...

-BB