The Sealed Cave is located in the Underworld and can only be opened with Luca's pendant.
This game's Sealed Cave is the Cave of the Gi, which is officially referred to as "Sealed Cave" in the menu in the area of descent. The cave is sealed behind the Gate of Naught in Cosmo Canyon and only Bugenhagen can open it. It is visited once in the game as part of the storyline.
The Den of Woe, known in the Japanese release as Mushroom Rock Road: Sealed Cave, is an optional cave located within the Mushroom Rock Ravine. It is sealed with ten Crimson Spheres.
The Sealed Cave is found in the Palmy Desert sands where the ultimate magic of Nuke is located. It was sealed by Lich and Sumo comes here in search of it.