Q. Why are there so many cemeteries near the border of Brooklyn and Queens? And what happens when they’re all filled up?

A. There are more than a dozen cemeteries near the line separating the two boroughs, in an area sometimes called the Cemetery Belt. More than five million people are buried in Queens alone, outnumbering those living there by more than two to one.

The reason for the concentration is simple: As the population of New York — then just the island of Manhattan — soared in the early 19th century, the city had trouble finding space for its dead. Churchyards and backyard family plots, long the burial locations of choice, were hemmed in by buildings and could not expand; instead, bodies were sometimes piled into shallow mass graves.

As a result, by 1822, unbearable odors were emanating from some cemeteries. To try to speed decomposition, one official had the yard at Trinity Church slaked with lime; he later wrote to a colleague, “The stench was so offensive as to cause several of my laborers to cascade freely.” (Yes, “cascade” means exactly what you think it does.) Burials were banned south of Canal Street the next year.