The previous owner of my home placed an underground drainage system for the rain gutters on this home. Now along what I believe is the path of this drainage there are several sink holes forming that vary from one foot wide to nearly three feet wide. I know sink holes will continue to wash out if not stopped so what do I need to do to stop this ‘washing out’ of sub-surface soil? I have been told to dig them out down to the drainage system and place sand and gravel at the bottom and place soil on top of that, is this the best approach or should I do something else? Please help before I lose my back yard! Thank you in advance!