The Door to Hell, as local residents at the nearby town of Darvaza have dubbed it, is a 70 meter wide crater in Turkmenistan that has been burning continuously for 35 years. In 1971, geologists drilling for gas deposits uncovered a huge underground cavern, which caused the ground over it to collapse, taking down all their equipment and their camp with it. Since the cavern was filled poisonous gas, they dared not go down to retrieve their equipment, and to prevent the gas escaping they ignited it, hoping it would burn itself out in a couple of days. Unfortunately, there was a slight miscalculation as to the amount of gas that was trapped, and the crater continues to burn to this day.
You can see it on Google Earth at 40°15′8″N 58°26′23″E