Somewhere in Darvaza, Turkmenistan exists a large pit in the ground; the contents of which have been burning for more than 40 years.
As it turns out, investigations took place in this area around 40 years ago to see if it would be a good place to set up a gas rig, but when the scientists found a lot of natural gas in the reservoir, they thought that by lighting it on fire, it would eventually run out and the flammable gasses would be no more.
Unfortunately, the Earth continues to fuel the flames they set with natural gasses from the ground, so there doesn't appear to be any end in sight for the flames, which continue to burn today.
The gasses are the result of organic material making its way into the ground and decomposing. The gasses are colorless, odorless, and poisonous, which explains why it was so necessary for the scientists to try and get rid of it. But for the mean time, they'll just have to continue waiting it out.