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?