The Fourth of July fireworks display in Washington, D.C. is a vast annual logistics exercise for the National Park Service. Dozens of security checkpoints, hundreds of porta-potties, thousands of fireworks are prepositioned, all ready for the crush of spectators that descend on the National Mall as darkness sets in on Independence Day.

One lesser-known but critically important bit of preparation involves fencing off a 91-acre safety zone around the launch tubes. During the show this area will transform into a Pompeii-like storm of falling embers and ash while the bombs burst in air above. Immediately after the grand finale, NPS employees dash out into the field to extinguish any small fires and collect hazardous material.

Today the expansive safety zone is spaced to offer an an abundance of caution to visitors, but they haven’t always maintained such a tight ship. In 1987 a woman seated nearby received third degree burns from a softball-sized flaming ember, three others were hospitalized, and 25 received Red Cross treatment on The Mall. The Washington Post interviewed a man seated 45 yards away who recalled that “it literally was raining ashes during the entire display. It was getting in people’s eyes and hair.” Still, as late as 1999 the Post was still reporting on the groups of children who delighted in “chasing the charred bits of firework paper that fell in a rain of ash.”

Security preparations began to approach their modern boundaries in 2004 after the completion of the nearby World War II memorial. The Park Service feared that falling ash would mar the new memorial stonework and covered the entire thing in protective tarping, and kept people at a safe distance with fencing. While the tarps were later deemed excessive, the fenced of firework safety zone is likely to remain a fixture of the Fourth of July Mall celebrations for the foreseeable future.