Secret levels

[Wuzzy]

In this post I will document all secret levels I have found so far.

Most secret levels are accessed by walking on an invisible path on the worldmap. If you want to find secret levels yourself, just go to any level or junction and try to go in a direction without a visible path.

Story Mode: Icy Island (as of v0.6.0)
Between Two Glaciers: Select the “Icy Valley” level on the worldmap and go North.

Story Mode: Forest World (as of v0.6.0)
Ancient Ruins: Select Crumbling Path and go South.

Bonus Island I
Area 42: Enter Matr1x' Sector and complete the first island. Use the teleporter at the end to teleport to the next island. Now go to the T-junction, then go East.

Bonus Island II
End of the Ice Age: Go to the T-junction that connects In Flanders Ice Field, Blind To The World and A Long Journey. On this T-junction, go East.
Life Support: Go the the T-junction that connects Firestorm, High Gravity and a dead end. Then go East.

Bonus Island III
The Toilette Zone: Go the the T-junction that connects “A hilly landscape”, “Crystal Skies” and “It's getting unfinishable” and go East. If you complete this level, you gain access to the forest section, a large number of forest-themed levels.
Flower Bonus: Select “It's getting unfinishable” and go South.

Hume's Rainbow Cave
There are 7 bonus levels. To unlock them, you must first complete the achievements level. To do so, collect all achievements, then just walk to the gate at the right.
In the level “Bonus levels gate”, you must enter a secret code. The secret code is 567418. Now your path to the bonus levels is free.

Yeti's Revenge
Collect the fish box in every level to unlock a path to more levels that lead to a boss.

Mattie's World
Special levels:
Time Warp: North of your starter igloo. You can warp to easier to more difficult versions of the levels here.
The Shop (for normal difficulty only): Go to the road between level 07 and 08 and go North.

Bonus levels (in normal difficulty):
Little Strange Day: Select “01 - A Day of Doom” and go West.
Where's Waldo: Select “05 - Dusky Dungeon” and go East.

Super secret bonus levels (in normal difficulty):
Oasis: Select “10 - Over the Great Wall” and go North, then East, then when it says “Um, are you lost”, press the action key, then go East again.
Ambassador Flathead's Airship: Select “10 - Over the Great Wall” and go North, then West.
Original Concept Toad Castle: Select “Toad's Castle” or “13 - Castle Escape” and go North.
Upside-Down Mine Shaft: Go to the road between level 04 and 05 and go East.