Minecraft wiki entry for Trapdoor_uses, associated text: Dig down until you find at least 3 solid non-gravity affected blocks (e.g. neither gravel nor sand) below the sand or gravel you want to clear (you only need to be one lower, but going an extra level deep to start reduces the chance of you running into a sudden drop in the level of the bottom of the sand). Clear out as much as you can one level below the bottom level of sand (so there's still something solid for the sand to sit on) in such a way that you can position trapdoors under the solid blocks. Make sure the trapdoors are closed and then just start breaking the blocks above the trapdoors and watch the sand all come tumbling down at once, breaking into objects galore!