Some fires are big enough to see from outer space, others burn for a very long time, but from what I could find, this is the oldest continuous fire in the world. Beneath an Australian mountain, a fire has been burning continuously since 4000 BC.
Way before Egyptians started building pyramids and in fact way before there was an Egypt, this fire was still burning quietly in Australia. To an observer on the surface, Burning Mountain would seem like a rather normal mountain - except for the foul-smelling steam; you wouldn't really suspect that the coal inside the mountain has been fueling a fire for the past 60 centuries.
No one really knows what started the fire, but I think it's safe to say we can count Billy Joel off the suspect list. Local Wanaruah people have been using it for thousands of years for heating, cooking, and to make weapons. It could have been a lightning, a forest fire or even spontaneous combustion. Another likely possibility is that the Wanaruah set it ablaze during their traditional rituals - either intentionatelly or on purpose.
The underground fire is estimated to be at a depth of around 30 m (100 ft). Interestingly enough, the fire isn't sitting in one place - it's moving at a rate of about 1 m (3 ft) per year. European explorers and settlers discovered it in 1828, and wrongfully believed it was caused by volcanic activity. Burning mountain, or Mount Wingen (which means “fire” in the local Aboriginal language) is now attracting many tourists, but it's also causing significant environmental damage.
Coal fires are surprisingly common in the world; an estimate puts the number of coal fires taking place on Earth at any given time at around 6,000. The American West is smoldering with coal fires from abandoned mines, and in fact, the entire landscape was shaped by these fires.
“Much of the landscape of the American West — its mesas and escarpments — is the result of vast, ancient coal fires,” writes Kevin Krajick in Smithsonian Magazine. “Those conflagrations formed ‘clinker’ — a hard mass of fused stony matter. Surfaces formed in this way resist erosion far better than adjacent unfired ones, leaving clinker outcrops.”
However, India and China have the biggest problems with burning fires, due to the huge number of abandoned mines.