CNN Travel has published an article about Yanar dag State Historical-Cultural and Natural Reserve, as well as the Atashgah Flame Temple in Azerbaijan.

Report informs that the article says these eternal fires are the reason  why Azerbaijan is called the “land of fire”. Venetian explorer Marco Polo wrote of the mysterious phenomena when he passed through the country in the 13th century. Other Silk Road merchants brought news of the flames as they would travel to other lands.

The article also states that Ateshgah played an important role in the development of Zoroastrian religion, which was founded in Iran and flourished in Azerbaijan:

"For a deeper insight into Azerbaijan's history of fire worship, visitors should head east of Baku to Ateshgah Fire Temple. 

Since ancient times, they think that [their] god is here,' says our guide, as we enter the pentagonal complex which was built in the 17th and 18th century by Indian settlers in Baku. Fire rituals at this site date back to the 10th century or earlier. The name Ateshgah comes from the Persian for "home of fire" and the centerpiece of the complex is a cupola-topped altar shrine, built upon a natural gas vent. A natural, eternal flame burned here on the central altar until 1969, but these days the fire is fed from Baku's main gas supply and is only lit for visitors," the article reads.

It adds that the temple is associated with Zoroastrianism but it's as a Hindu place of worship that its history is better documented.