The always excellent Smithsonian history blog Past Imperfect posted about Chilean witches with headquarters in a secret cave:
The most important of the warlocks brought to court in 1880 was a Chilote farmer by the name of Mateo Coñuecar. He was then 70 years old, and by his own admission had been a member of the Righteous Province for more than two decades. According to Coñuecar’s testimony, the society was an important power on the island, with numerous members, an elaborate hierarchy of “kings” and “viceroys”—and a headquarters located in a vast cavern, 40 or more yards long, whose secret entrance had been cleverly concealed in the side of a ravine. This cave (which Chilote tradition asserts was lit by torches burning human fat) was hidden somewhere outside the little coastal village of Quicavi, and was—Coñuecar and other witnesses swore—home to a pair of monsters that guarded the society’s most treasured possessions: an ancient leather book of magic and a bowl that, filled with water, allowed secrets to be seen.
That's pretty ripe for write up as an adventure.