Does anyone knows a way to detect underground structures? Let me explain my problem. I own a very old house. The kind of skeptic tank is tough by most elder neighbors to consist of two cavities dug in the ground. I do see two quite big spot of lawn (a few feet wide each) that are "falling down" since I bough the house. Each year, the two depressions seem to lower even more. I think these are kind of covers (probably wood) and I'm afraid they will break at one point as they are obviously very old. As I have no clue of the size of the cavities, it's hard to figure out how close I can safely get. They must be quite big since they are receiving poop and greywater for 90 years now. Any advice on how I should proceed to determine the size of the cavities so I can establish a safety perimeter and then fill the cavities or build new covers?