I noticed that Milo Dolezal recommended lowering a clean out plug below the surface to prevent damage. I did that in April 2007, covered the clean out with a metal dish so I could find easily, and covered with sod. I sold the house in May 2008. In April 2010 a sink hole developed in the new owners yard at the water meter and clean out. They paid a plumber $17,000+ to repair a leak at the water meter and replace about 15 feet of sewer line between the house and Y-connector at city sewer. Now they are threatening to file a lawsuit if I don't pay half of the bill. They are claiming that I should not have covered the clean out and because the clean out was covered it could not be inspected at time of purchase which would have revealed the problem prior to purchase of the house. Basically, they are saying that the problem existed in 2008 and that I disguised the problem. The house is about 10 years and neither of us ever experienced water or sewer problems even after the sink hole appeared. Additionally, I have gotten estimates from reputable plumbers indicating that the repairs should have cost $3000 to $3500. I need some help please.