The Gate Of Hell
The mystery dates back to 1971, when Soviet geologists began drilling the Turkmen desert in search for oil. However, little did they know that they were actually drilling on a huge reservoir of natural gas, and before they knew it, their equipment was swallowed by the earth, forming a massive pit.
Scientists soon realized that natural gas was leaking through the crater, and they tried to combat the leak by intentionally setting the crater on fire, hoping it would soon burn out. Soviet scientists figured that the fire would only last a few weeks, but 5 decades have passed, and it doesn't seem like it'll be stopping any time soon!