Minecraft wiki entry for Altitude, associated text: This is not to be confused with altitude when otherwise expressed as the top face of a block layer, where the lowest block that can be placed is at layer -63 and has a Y-coordinate of -63. For example, sea level is at layer 63, while clouds appear at layer 192. This is akin to calling the ground level story the "first floor" (top face of a block layer), rather than the "ground floor" (bottom face of a block layer). The player can press F3 to see the Y-coordinate of the top face of a block they are on top of.