It is a very extensive geological formation, resulting from the formation of a lava tube in the interior of the island, included in the Serra de Santa Bárbara and Mistérios Negros Natural Forest Reserve.

Furnas do Enxofre is located in the parish of Porto Judeu, in the Municipality of Angra do Heroísmo, on the island of Terceira, in the Azores. They constitute a fumarolic field, that is, an area that comprises several outlets of aggressive volcanic gases, at different temperatures, some quite high.