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If you place water to completely flood the floor area around a monster spawner, will it prevent monsters from spawning? Or can monsters still spawn in water? The wiki says:

It tries 4 times to place the mob at a random position in an 9 x 9 x 9 area, and gives up on placing that mob if it cannot find a valid position after the 4 attempts.

So is a block with water flowing over it valid? I'm asking both from the perspective of neutralizing a dungeon (making it safe) and from designing a dungeon trap (a floor with water flowing over it would direct mobs to the trap, but would it also prevent them from spawning in the first place).

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The rules for "normal" spawns do not seem to apply fully to Mob Spawners. A dungeon with a completely flooded floor is not safe. Apparently the random block in the 9x9x9 cube can be air.

Here's a flooded dungeon, and here's the same dungeon after I turned the difficulty to Easy (and removed the torch). The spider spawned in the water and climbed up to the ceiling.

Edit: Test repeated with source blocks, and a torch for better visibility.

Setup

peaceful

Result: still not safe

easy

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    Excellent, this is the first example I've seen which I'd call "indisputable". (Usually, the spawner itself is left uncovered.) If you're willing, I'd love to see the same test repeated, but with each block covered with source water rather than flowing water. I've often wondered whether that makes the difference.
    – Ben Blank
    May 26, 2011 at 22:12
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    Can do! So I did. I also removed a ledge I overlooked in the previous test and left the torch for better visibility, as it doesn't give off enough light to disable the spawner. The only non-source blocks are those coming from the water block covering up the spawner; the floor is all source blocks.
    – a cat
    May 27, 2011 at 14:51
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A Mob Spawner can definitely still spawn with a room covered in water. I have created a skeleton trap with completely water covered floor to sweep the skeletons down into a pit to drown, as well as sweeping the items out to me.

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Mobs can't spawn in water, even from Monster Spawners! The special rule of spawners is that they can spawn mobs in midair; their spawns still require appropriate (lack of) lighting and an empty space, but they do not require an opaque block underneath like normal monster spawns do.

It may appear that mobs are spawning in water since they immediately fall in, but if you were to completely flood the room (no air blocks anywhere in the volume, only water blocks), no mobs would spawn.

For purposes of creating traps, this means the most efficient design is to make sure the 8 × 3 × 8 space around the spawner (and one more layer upward if spawning tall mobs, i.e. skeletons or zombies, and one more block sideways if spawning spiders) is empty air, and place water two blocks (or lower) below the spawner to carry mobs away.

I have reviewed the current version of the Monster Spawner page's Spawning Behavior section at Minecraft Wiki, and it appears to have its facts correct; it covers what I have mentioned here and more.

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My son and I have created several mob-spawners with completely water-covered floors and trenches to sweep them into a shaft (which kills them on the fall). The height of the room is two blocks, room size is 18x18, and the trench is 2x2, forming a cross with a pit in the center and water sweeping mobs directly into the pit. Not sure why Kevin indicated that it could not be done. I use Xbox One, and my son plays on both Xbox One and PC. It seems to work similarly on both systems. Beyond these, we have not tested extensively, but I can say that it definitely does work.

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