The Magic Cave is the secret access route to Magus's Lair, located on the easternmost tip of south Zenan. During the Mystic War, it was used to attack Zenan continent; a few villagers in Dorino actually saw it in use. Its exterior was sealed by a magical spell; when Crono and Frog assaulted Magus's Lair, the spell was broken when Frog, in one of the most powerful scenes in Chrono Trigger, charged up the Masamune and split the cave open. Ironically, the cave's easy access allowed the heroes to storm the Lair and defeat Magus, effectively turning the tables on Ozzie and his henchmen.
Translation: Demon Cavern