It's possible to get under Celestia and reach the out of bounds mountain in Fontaine, but what do they look like?
Tutorial:    • How To Do The Out of Bounds Glitch in...  

0:00 - Note
0:18 - Fontaine Mountain
2:38 - Celestia

Music:
Drenched Bluff - Pokemon Mystery Dungeon (Explorers of Sky)
Revelation Mountain - Pokemon Super Mystery Dungeon