This location is the last location that you visit before completing the game. It is rarely referred to in the game but it is the zone (or two zones) that follow after you reach the bottom of the Northern Cave.
Once you have reached the “Inside the Planet” location there is no turning back. Refer to the Northern Cave walkthrough section for details on how low you can go before you reach this unreachable location. You can refer to the “Inside the Planet” walkthrough section by checking out the last page of the walkthrough where you will face off against Sephiroth.
The map below shows the location for “Inside the Planet” on the World Map which is in the same location as the Northern Cave:
