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.
|Level number||Level name||Preview||Description|
|D-1||Swooper Cellar||This level takes place in a cave with lots of Swoopers.|
|D-A||Carapace Catacomb||This takes place in a catacomb, as the name implies. This has Halloween as the goal pole area.|
|D-Fortress||Toxic Fortress||This level takes place in an underground fortress filled with poisoned water. Some of the floors/walls drop upon touch, so be careful. There is no boss in this level as with all of the lettered world fortresses.|
|D-switch||Red Switch Palace||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.
- The music of the levels is a remake of the underground theme from Yoshi's Island.