Where is the world's oldest continuously-burning underground fire?

Wingen Mountain, near the town of the same name in New South Wales, Australia, is commonly known as ‘Burning Mountain’ thanks to the coal-seam fire burning some 100 feet (30 m) below. The fire is the oldest known coal fire, and it is estimated to have been burning for approximately 6,000 years.

Underground coal fires are actually fairly common; there’s a fire that’s been burning in the abandoned town of Centralia, Pennsylvania, USA, since 1962. Once they start, they are nearly impossible to extinguish due to the insulation from surface elements.

No one knows exactly what started the Burning Mountain fire; hypotheses include forest fire, lightning strike, or even spontaneous combustion. The only sign of the fire is sulfurous smoke on the surface and areas which have caved in due to heat and toxic gases.

European explorers and settlers to the area initially believed the smoke, coming from the ground, was volcanic in origin. It was not until 1829 that geologist Reverend C. P. N. Wilton identified it as a coal seam fire. The fire is generally moving in a southerly direction at a rate of about 3 feet (1 m) per year.