Fire in the hole
In 1971, them crazy Soviet geologists were doing what they do best: drilling for gas, when they discovered a large chamber of gas, and it seems from varying reports that either there was something of an explosion and the equipment fell "deep, deep underground", OR they lit it to stop poisonous gases escaping. Either way it's been burning for 37 years now.
Personally I prefer the second explanation, as I like the image of one guy dropping a match into a hole and running like fuck.
Also, I was thinking that some marshmallows and really long sticks would make a great addition.
More to be found here