In 1971, Soviet geologists prospecting for oil and gas in the the middle of Turkmenistan drilled into a large cavern filed with natural gas, collapsing the ground around their drilling rig and creating a hole more than 200 feet wide. The geologists decided to prevent the poisonous natural gas to seep out into the surrounding area by lighting it on fire, sure that the gas would burn itself out in a few days.
The hole is still burning today and was nicknamed the "Door to Hell" by local residents, for obvious reasons. The photo above gives you an idea of how closely it matches the historic definition of hell (for a sense of scale, check out the people standing near edge of the hole) but you really need to see video to get the full picture. The ground is literally burning and has been for more than 40 years.
It reminds me of Centralia, Pennsylvania., home of an underground coal fire that's been smoldering (and slowly burning up areas of town) since 1962. Why is that all the worst man-made energy disasters involve fossil fuels?