This is a 328 foot wide hole that has been burning for 38 years. In the early 1970s, the Soviets were drilling a hole when they (oops) punctured a large natural gas pocket. The ground collapsed pulling the drilling equipment with it. Poisonous gases leaked out and to prevent a catastrophe, they set it on fire.
Need to enter the underworld for any reason? Here you go. It is located in Turkmenistan.