The Great Cavern of Wonders is a location in Crisis Core -Final Fantasy VII-. It is a long, deep series of caves that hide the game's superboss, Minerva. It is visited in Missions, and the mission series 9-1-1 to 9-6-6 covers Zack's exploration of the cavern.
Geographically, it is in the Icicle Area of the Northern Continent next to the Northern Cave. While its exact location is not known, its placement next to the Northern Cave suggests it is either within the North Crater or very near to it.
Shinra Electric Power Company unearths the entrance to the cavern near the Northern Cave, and sends Zack to investigate when their expedition team does not return. The cavern is full of powerful monsters, and it becomes apparent that others are also exploring the area: Genesis Copies and Wutai remnants.
On deeper levels something causes the Shinra machines sent into the cave to go berserk and unable to distinguish friend from foe. The caves go deeper than Shinra had presumed and only the strongest Genesis Copies and Wutai troops have been able to survive the trek.
By Mission 9-5-5, only Zack is strong enough to continue deeper into the cave, the Genesis and Wutai soldiers having been cleared out and other SOLDIER operatives unable to survive the journey. Shinra begins to detect a mysterious energy source from within the cave, strong enough that machines can take on biological properties, and asks Zack to continue to the lowest level to find the source. In Mission 9-6-6, Zack reaches the lowest level and finds Minerva, engaging her in battle.
While the rest of the cave uses generic mine and cave-type maps, the area where Minerva is fought is a replica of the Depths of Judgement area where Emerald Weapon can be seen in the background. Emerald Weapon being seen here makes more sense than in the Banora Underground since the Weapons are resting in the Northern Cave at this point of time, a cave system located next to the Great Cavern of Wonders.