Most people ambling along city sidewalks don’t realize that what they’re walking on is not solid ground, but likely the roof of an underground vault. Like all building components, below-grade spaces require periodic maintenance to resist the ravages of time and the elements. Sidewalk vaults tend to be used as mechanical rooms or storage space, so they garner less attention than those parts of the building people actually see. Moreover, much of the deterioration may not be visually evident, since it is often concealed behind masonry or fireproofing that appears intact. Neglecting the space below city sidewalks can spell disaster for building owners and the public should the structure collapse. In overcrowded cities, sidewalk vaults are also valuable real estate that is often overlooked.