Jwala Ji ( Jwala Devi and Jwalamukhi Ji.)  is a Hindu Goddess.The physical manifestation of Jwala Ji is always a set of flames burning off natural gas,and the term Jwala means flame in Sanskrit.

Historically, shrines dedicated to Jwala Ji were based on fissures from which natural gas seeped by itself. The number of flames is usually either seven (for the seven divine sisters) or nine (for the nine Durgas) Several schools of Buddhism also share the symbolism of a seven-forked sacred flame. Jwala Ji shrine is located  in Jawalamukhi town of the Kangra district of Himachal Pradesh state, about 55 kilometers from the town of Dharamsala.

According to the legend, when Sati's body was divided into 51 parts, Sati Mata's tongue fell here. The flames/ Jyotis are the representation of the same. Some say that Sati's clothes fell here. When they fell they were on fire. The fire hasn't blew off. Jwalaji (flame) or Jwala Mukhi (flame mouth) is probably the most ancient temple besides Vaishno Devi. It is mentioned in the Mahabharata and other scriptures. There is a natural cave where eternal flames continue to burn.

The Legend

A cowherd found that one of his cows was always without milk. He followed the cow to find out the cause. He saw a girl come out of the forest, drink the cows milk, and then disappear in a flash of light. The cowherd went to the king and told him the story. The king was aware of the legend that Sati's tongue had fallen in this area. The king tried, without success, to find that sacred spot. Again, some years later, the cowherd went to the king to report that he had seen a flame burning in the mountains. The king found the spot and had darshan (vision) of the holy flame. He built a temple there and arranged for priests to engage in regular worship. It is believed that the Pandavas came later and renovated the temple
Reference: https://en.wikipedia.org/wiki/Jwala_Ji

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