The Depths
"The Depths" are any positive levels that are located outside of "The Main Nine".
Levels from The Depths can be linked to levels from The Main Nine though, like how Level 0, 12 and 13 are very closely linked. This is a rare example though. You'd usually need to get back to Level 8 through Level 9.
Level 9 - "Nocturnal Suburbs"
Level 10 - "The Bumper Crop"
Level 11 - "Windows"
Level 12 - "Sublimity"
Level 13 - "Ligneous Desolation"
Level 14 - "Pink Paradise"
Level 15 - "Snow World"
Level 16 - "Mindless Hills"
Level 17 - "Uncomfort"
Level 18 - "Remote Island"
Level 19 - "Pumpkin Patch"
Level 20 - "Library"