I've recently noticed that a small sink hole is developing about 3 feet away from our septic tanks. it is over the direction where the field drain would go. The grass grows great there, but is sinking. I dug it up over the weekend, but don't really know what I am looking at. The field drain was replaced about 10 years ago. The house was built in 1962. Let me know what you think is happening and how expensive of a repair it might be.