Why is Azerbaijan called the land of flames?

Azerbaijan is called "The Land of Fire" or "The Land of Eternal Flames" due to the fact that the ancient Persian religion, Zoroastrianism, was "big" there. Zoroaster, the prophet of the religion, was born in Azerbaijan, or as they called it Azarapadegan. Some say the origin of the name "Land of Eternal Flames" comes from the fact that natural gas would seep out from hills and the fire would essentially be eternal.