Nate Zigler wrote:

I've lived in this place for 21 yrs. When I first moved in, there was a
huge indentation in the back yard, approx. 30'50'x8" deep.

You don't mention your general geographical location, and aioe doesn't
provide any clue in your headers. That makes it difficult to say what is
probable versus merely possible.

All utilities including sewer run out front, so I kind of figure it's not
from that.

In addition to the possibilities already mentioned, it could be a
collapsing cistern or old septic system. Has your home been on city water
and sewers from the day it was built?

As a side note, about 10 yrs. ago, there were some properties about 20
miles from here, which the houses fell into some old mineshafts. The
shafts were never charted, so no one knew they were even there. Because of
this, I had mine insurance added as a rider to my policy.

Very wise move.

--
Tony Sivori