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. I
brought in 40 yards of topsoil to fill the spot in. My yard isn't
flat, but has a slight slope all the way around.

About 6 yrs. ago, I brought in another 10 yards, because of a the dip
reappearing.

Now, I have the same area of 30'x50, but it's about 24" deep. It's
looks like a sinkhole!

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

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.

It's now gotten to the point of getting me spooked.

Who do I contact, or what's the first steps, getting in touch with the
proper agencies?

OK, what's the topography - flat land, on the side of a hill, base rock
chalk/ limestone? Others have made comment about the possibility of
sewers (could be an issue in a chalky/ limestone area). Water mains?
Unlikely in that the supplier is likely to note a loss resulting in two
foot of soil loss in 6 years, (at least here in the UK).

How old is the property? My first thought when I read the start of your
thread was former mining - which you referred to later on. What do you
know of the former use of the area?

Just thoughts; not a solution I'm afraid.