Minecraft wiki entry for Command_blocks_and_functions, associated text: The coordinates given are ~0 ~-1 ~-3 (so 3 blocks in the negative Z direction from the commandblock's current position and 1 block down). This presumably will imbed it in the surface of the floor. Keep in mind that a spawners "SpawnRange" setting only applies horizontally not vertically. Vertically it will only spawn at the same height or 1 lower or higher, so imbedding it in the surface of the floor cuts the available spawn area by 2/3rds (but can help hide it). Naturally occurring spawners have their height cut by 1/3rd due to appearing 1 block above the floor instead of 2.