Ok i just got a new client for this year, and i went to check out her yard, there's a sinkhole about id say 4 feet X 4 feet. maybe a little less, she says she has been living there for years and has never had any trees, septic, lines, nothing in her yard. the client also had someone come in a "few" years back and fill the same sinkhole but it was not as severe as it is now. it seems to be maybe 1 foot deep. but i just checked one area. what can be causing this?? its in the middle of her backyard. what can i do to fill it ?