Sharraxaadda

The retaining wall along the sidewalk between these two houses has sunk several inches on the side toward 206 South Belmont Avenue. There is a hole at that end of the retaining wall that is at least 3' deep. I know, because I stuck a yardstick in to test its depth. There is also a small hole across the sidewalk near the street. I'm afraid the hole may go completely under the sidewalk. I don't want it to cave in and injure someone.