Which of the following is NOT a potential sign of unmarked underground utilities?

The correct answer, which identifies something not typically associated with unmarked underground utilities, is narrow depressions in the ground. While narrow depressions can occur for a variety of reasons, they are not a specific or reliable indicator of underground utilities being present. Most utilities would not cause a noticeable depression to form at the surface unless there were significant issues such as leaks or ruptures that could lead to a sinkhole or a collapse, which would be more extreme cases.

On the other hand, sewer clean outs and gas meters are physical access points or indicators that facilitate maintenance and monitoring of underground utility systems. These visible indicators suggest that there are buried utilities in the vicinity, as they provide necessary access to these services. Electricity lines, similarly, have associated infrastructure or accessories that are visible above ground, indicating their presence below. Unlike these recognizable utility markers, narrow depressions may arise from numerous natural causes unrelated to underground infrastructure.