I live in a new subdivision that used to be farm land. Last year a couple small sink holes showed up in an area of my back yard. Small is relative...it ended up being a 2' wide by 2' tall 'tunnel' about 10' long where some areas of the turf had caved in. This 'tunnel' then turned down and disappeared into the ground. I filled it with a a combo of rocks and fill that I still had around from cleaning my yard before seeding. In talking with neighbors this spring, I found a significant number that also had these types of sink holes. What is the cause? Since this is a new sub, there could have been a lot of fill brought in and who knows what buried. I guess if a log or other organic matter was buried that then rotted and collapsed. I did find another extension of this in the same area this spring but I have yet to cut the turf and investigate/fill. They seemed to show up after an excessively wet period but this could be coincidence, who knows. Thanks!