For the past 6 of 9 since I had a perennial rock garden planted I have noticed one area where a sink hole or cave below one of the large boulders has been forming. The slope there has been sinking over the years and when I have tried to fill it up with dirt, the size of the hole seems to be almost cave-sized. The more I have dug down the vastness of the empty space down there has been a shock to me. I’ve not been able to find a solution and just let it go last Summer but want to fix this problem. I’ve had landscapers fill it with dirt and tell me it is erosion from water. Others have told me it is a sink hole that you will have to fill forever. A garden center told me it was gophers one year and I put out something to deter but not poison them. I tried calling the corps of engineers at Ramsey County and got no response. Just now, a landscaper handed me a brochure and I showed him the spot and he said it was rabbits hibernating but I know they don’t do that. I am afraid it is going to collapse when I am out there gardening and also worried that my beautiful garden and work is eventually going to be destroyed. Who can help me figure out what is really going on? I know the company that put the garden in was very expensive and were not at all helpful when I called them back with questions or complaints when many things died the next year including a tree so I don’t think it would be worth calling them. Is this a horticulture engineering problem to help solve? And if so, how do I find one?