The Darvaza gas crater, known to locals as "Door to Hell" or "Gates of Hell" is located in the Karakum Desert of central Turkmenistan (about 150 miles from the nation's capital).
Also known as Derweze, the pit lures tourists and unsuspecting desert critters: "reportedly, from time to time local spiders are seen plunging into the pit by the thousands, lured to their deaths by the glowing flames." Smithsonian Magazine has an awesome photo gallery.
As with much on our planet, turns out this site is a man-made hell:
The Derweze area is rich in natural gas. While drilling in 1971, Soviet geologists tapped into a cavern filled with natural gas. The ground beneath the drilling rig collapsed, leaving a large hole with a diameter of 70 metres (230 ft) at 40°15′10″N 58°26′22″E. To avoid poisonous gas discharge, it was decided the best solution was to burn it off. Geologists had hoped the fire would use all the fuel in a matter of days, but the gas is still burning today. Locals have dubbed the cavern "The Door to Hell".
Here's a Google Maps view. Users who've visited the bizarre site have uploaded more photos there.