Swamp gas.
I drove my girlfriend and myself and another couple down to a dirt road famous for the Cogdell light.
We parked for a while and then gave up. As we were leaving, it followed my car for a while down the dirt road.
Since cars back then didn't have spark arresters, I wonder if the methane from the swampy area nearby was drawn into the vacuum created behind my car and then ignited by the auto exhaust.
Scared the hell out of all of us.
It was big (about 10ft in diameter)...roiling flames.