Minecraft wiki entry for Terrain_features, associated text: Hollows are the opposite of floating islands. They look like caves, but they have nothing to do with cave generation (although they may intersect with them). When there are many overhangs, they close together and create a hollow. They have exactly the same floor as the terrain above, depending on the biome that they are located in, unlike caves. Hollows have no specific floor. Grass blocks can generate inside too and survive without light. When they generate under the sea level, they are filled with water. They are extremely rare in the default world. Can be found far more commonly in certain customized worlds.