Minecraft wiki entry for Falling_blocks, associated text: Create ghost blocks. With the addition of the NoGravity tag, a falling block can float in the air, leaving the player the thought that a solid block is there, but in fact that block can be passed through. This can be used for custom maps to hide stuff or to create secret ways. However, these ghost blocks disappear when enough time passes, so one will need to make a Command Block clock or use a Repeating Command Block to respawn them.