Minecraft wiki entry for The_second_day, associated text: You may see shelves of "floating" sand or gravel, which the game generated without support (that is, a player couldn't have placed them like that). Be careful around these, as they are natural traps: Mining them or placing any block next to them sets off a chain reaction that can bring down the whole shelf. Sand shelves appear in deserts, while gravel shelves (or ceilings) can appear underground. When seen from underneath, sand and gravel shelves drop "falling dust" particles. Always be careful not to mine or place any blocks touching them.