Take a look at the picture above. It is from a place called the "Door to Hell", which is situated in Uzbekistan, near the small town of Darvaza (or Derweze). Why it is called "Door to Hell"? Because there is a fire continuously burning since 1971! So, locals have named the cavern The Door to Hell.
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. As of 2009, gas from the cavern are burning uninterrupted.
Take a look at some more pictures of the "Door to Hell". Plus a video.
Take a look at the "Door to Hell" video: