Centralia, Pennsylvania: Hell on earth!
Centralia is a town located in the heart of the anthracite mining region of Pennsylvania. In May 1962, a fire in a trash dump accidentally ignited the coal mine under the town. Attempts to put out the fire over the years have been unsuccessful, and the population of 1,100 residents has since dwindled to 21.
On the south side of Centralia is a section of four-lane divided highway which was closed in the '80s. There is a huge crack in the southbound lanes, with smoke billowing out of it from the underground mine fire, which continues to burn even today.
Centralia is not the longest burning fire but is interesting nonetheless. The town was immortalized in the movie Made in U.S.A., which features footage of the town when it was being abandoned in the 1980s.
Update: According to the 2000 Census, there are 16 housing units in Centralia, only 10 of which are still occupied.