Minecraft wiki entry for Mining, associated text: If players are mining in an area with sand or gravel, like a desert biome, they can break the sand or gravel blocks and see if any others fall. If any does fall, they know there is something beneath. This can be rather dangerous if blocks fall beneath players, but they can place ladders or dig their way out. If done during the day, hostile mobs will mostly burn or go neutral in the sun, but as always, watch out for creepers. Also, the fallen sand or gravel can cover ores. Another event is that it is possible that there is lava under the sand. This is most common under mountains. There could also be a dungeon under the sand, so be prepared to fight or retreat.