The "Gates of Hell"
In the hot, expansive Karakum desert in Turkmenistan, near the 350 person village of Derweze, is a hole 100 meters (328 feet) wide that has been on fire. For over 40 years years it has constantly been active. This hole is known as the Darvaza Gas Crater or the "Gates of Hell" by locals, the crater can be seen glowing for miles around.
The hole is the outcome not of nature but of an industrial accident. In 1971 a Soviet drilling rig accidentally punched into a massive underground natural gas cavern, causing the ground to collapse and the entire drilling rig to fall in. Having punctured a pocket of gas, poisonous fumes began leaking from the hole at an alarming rate. To head off a potential environmental catastrophe, the Soviets set the hole alight. The crater hasn't stopped burning since.
Though little information is available about the fate of the Soviet drilling rig, presumably it is still down there somewhere, on the other side of the "Gates of Hell."
: I'd go with number 2.
@Kai "the spy"
: I'd go with 2,3 and 4, in that order. Didn't help much there now did I?