The Caves of Boreth were a series of interconnected lava caves on the planet Boreth. According to the Klingon sacred texts, Kahless the Unforgettable indicated that he would reappear in the lava caves on Boreth. In 2369, Worf was in the caves when Kahless appeared to him, much as the prophecy had suggested. (TNG: "Rightful Heir" ; DS9: "The Way of the Warrior")