It has been burning for over 40 years! And this is no small pit, the diameter is 230 ft (70 metres)!YouTube
The Derweze area is rich in natural gas. While drilling in 1971 geologists accidentally found an underground cavern filled with natural gas. The ground beneath the drilling rig collapsed, leaving a large hole with a diameter of about 50-100 meters. To avoid poisonous gas discharge, it was decided to burn the gas. Geologists had hoped the fire would go out in a few days but it has been burning ever since. Locals have named the cavern The Door to Hell.
Next to capturing the gas, flaring is safer and friendlier to the environment than releasing the methane into the atmosphere as methane is a relatively potent greenhouse gas with a high global warming potential of 72 (averaged over 20 years) or 25 (averaged over 100 years).