Minecraft wiki entry for Traps, associated text: Explanation: The bubble columns keep the sand on the same level as the cobwebs, so it just looks like slightly darker sand (entity and block shading are different) but sand entities don't collide with players or mobs, so you get stuck in the cobwebs and you need to use a sword or shears to break them, and you have to hit through the sand entities. You could also break a block at the edge of the hole.