About a week ago i found a small sink hole (which is now much larger after playing with it) right between my front lawn and my sidewalk. Upon further inspection i realized that it was just next to the marking on the sidewalk where the main house drain connects into the sewer. The main drain is about 9' below grade and the hole is only 4' deep. I bought the house 2 years ago, and i learned from the neighbor across the street that the previous owner had recently connected to the city sewer - just months before the house was up for sale in early 2010. I was also told by my neighbor that the old cesspool had collapsed. The cesspool was a few feet back from the current hole. I have no issues with any of my drains or any back up in the main drain pipe. About two weeks ago i found a leak in my underground irrigation system. however, the leak was about 6' to 7' feet from where the hole is and there is no evidence of any underground cavity connecting the two. So here are my theories:

1. soil was not compacted properly after installing sewer pipe or repairing collapsed cesspool

2. cracked sidewalk caused water run off into the earth below it washing it away

3. sprinkler system leak or just too much water on lawn.

4. organic material buried under sidewalk decomposed over time.

please help! i would like to get some opinions before filling up hole.

Thank you