Minecraft wiki entry for Safe_home, associated text: If you discover a natural cave with a roof of sand or gravel, be extra careful. Even if you don’t choose it to become your new home, as soon as you place or remove a block near it, the loose sand or gravel will suddenly be affected by gravity and fall down, suffocating you, flooding the cave with lava or water and/or allowing mobs to come in from the dark cave/night above.