The Magic Cave is a location in Chrono Trigger that appears in 600 AD. Before acquiring Epoch, it is the only pathway to the Fiendlord's Keep. This passageway links the southernmost peninsula of Zenan, to a neighboring island. The water around serves a natural barrier to the fortress beyond.
After restoring the Masamune with the aid of the rare and powerful mineral, Dreamstone, Crono, Frog, and the others set out to find and slay Magus, leader of the Fiends and a dark and powerful mage. Upon arriving, the cliff face seems impenetrable and too steep to climb. As the characters search for an entrance, Frog reminisces on the death of his old friend, Cyrus (to whom he was a squire) and the events that led to his transformation into an amphibian humanoid. Subsequently, Frog unsheathes the Masamune, recites an incantation, and slices a gouge into the rock face. The slabs of stone separate cleanly and enter the bowels of the cavern. The ease by which the rock splits indicates that the stone itself contains magic or a magical spell was cast upon it, as the beings and objects of magic fear the Masamune and refuse to be in its presence. During the war against the Mystics, the cave was used as a base of operations as well as a channel for magical attacks. Many citizens in Dorino claim witness to this.
After battling through the monstrous inhabitants, the party reaches the island housing Fiendlord's Keep. Due to the impenetrability of the rock face, Great Ozzie and his minions were thrown off by the arrival of Crono. They believed the Masamune lost, broken, no more; and they did not expect the party to be infused with magical powers similar to those of the Enlightened Ones. Sacking the citadel was fairly easy from that point forward.
The cave disappears after 600 AD. In its shadow, only meters away stands a pyramid of mystical energy. Near this area is the tomb of Cyrus, known more commonly as Hero's Grave. In 2300 AD, Keeper's Dome and Mountain of Woe are roughly positioned near this location. Plate tectonics moved the continental shelves, which explains the severe drifting of landmasses between the eras, so it is likely the continent with this cave drifted to this approximate position. The proximity of Magic Cave, Cyrus' Tomb, and the other structures strikes a shocking similarity. These are places of mystical power and energy, so perhaps, the save itself formed on, and these structures were built upon, this site indicates this fragment of landmass has a mystical property of its own.