A Vertical Flame Diversion is a chimney that can be found at the back of a cave which is used to stop caves from filling with smoke when a fire has been lit. One can be found in the Mortmain Mountains, in a cave used by both Volunteer Feline Detectives and the Snow Scouts. This particular Vertical Flame Diversion, which has footholes chiseled into one of its sides, also serves as a passageway to the hallway leading to the first Vernacularly Fastened Door. These footholes were discovered by Quigley Quagmire after his reading of Remarkable Phenomena of the Mortmain Mountains.
It is also possible that they used a Vertical Flame Diversion to alert fellow volunteers of a fire, then allowing I the trapped volunteers to escape.