Sharraxaadda

There are two holes adjacent to the sidewalk. Over a number of years the holes have appeared and neighbors have filled them with sand and stone; but after a year or two they reappear. One time two large bags of sand and a bag of stone was needed to fill the hole. For a long time the theory has been that it must be a rodent, but there is no evidence or signs of digging. It is now the deepest it has been extending under a large portion of sidewalk. We are concerned that someone will accidentally step in the hole or that the sidewalk might collapse. Since this is a city sidewalk, we will appreciate your attention & solution to the problem. Thank you!