Crystal Caves, or World D, is the fourth secret world of Newer Super Mario Bros. Wii. It is unlocked by completing the secret exit in World 5-8: Magma Iceburrow. This world has two exits. Completing the fortress will unlock a shortcut to World 6-3: Fog Cemetery. However, completing World D-A will unlock a shortcut to World 6-6: Cawcaw Treeway.
Crystal Caves is a violet cave that has many underground enemies.
D-1: Swooper Cellar
D-2: Toxic Wigglershift
D-3: Spine Springs
D-A: Carapace Catacomb Unlocked by the secret exit in toxic wigglershift and will exit out to world 6-6.
D-Fortress: Toxic Fortress
- The Red Switch Palace can be accessed via the secret exit in 5-5. If the Palace is reached before Crystal Caves proper is unlocked, its world name on the map will read "???".
- Challenge House H can be accessed after completing D-1.
- The Crystal Music House can be accessed after completing D-A.