Minecraft wiki entry for The_second_day, associated text: Both sand and gravel, unlike most blocks, are affected by gravity: If either block is placed without something directly beneath it, or if whatever's beneath it goes away for any reason, it falls to the ground below. If it falls on you such that your head is buried, you take suffocation damage until you get free or mine your way out. They can also suffocate mobs, which can be useful for traps.