toller wrote:
I have had two like that because buried over tree trunks have decayed out.
I packed dirt in, and expect to have to do it again in a couple years.

Naturally your problem could be entirely different, but it might be the
same.



I once found a hidden hole in yard, most likely from a stump that rotted
away. Our property was filled by landfill about 40 years ago
(waterfront in Florida). We also had erosion problems along the seawall
that created tunnels beneath the sod, which could have been a hazard if
someone walked on one and it collapsed.

Your city building department or insurance company may be interested in
taking a look.

One oddity I noticed during our seawall repairs was an area of clay
soil. I've seen "clumps" of it that hang together along the beach where
most of the soil is sand. When some new erosion problems appeared, the
contractor tested drainage by running water onto the clay area, and the
water drained laterally and out through the seawall about 20' away.
Apparently the top of the clay was too compact and sloped toward the
seawall, so the water drained off more rapidly than it could drain
through the clay. This link is to an article that illustrates sinkholes
rather nicely:
http://www.mgs.md.gov/esic/fs/fs11.html

I lived in a neighborhood in Florida with major sinkhole disasters.
Good time to find out whether your insurance covers them. 100 year old
house? I'd be inclined not to panic.