Minecraft – Can slimes spawn in midair

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Do slimes need some floor to spawn in their spawn chunks, or can they spawn midair if I remove all the stone?

The Wiki is inconclusive, the notes marked with [citation needed] and [Verify]. A discussion from '11 notes they can spawn in solid rock (and die immediately) just fine.

My rough plan for an automatic slime farm is: dig a wide hole to bedrock at slime-spawning chunk using TNT-loaded dispensers, funnel them to a deep pond using water, cover the floor of the pond with hoppers. Slimes would spawn in midair above, fall, drown, and the hoppers would then collect the slime balls. With vertical walls and all side corridors (and surface) well lit, no other mobs should spawn.

That's the plan. Now the question is: will it work? Or do I have to build a bunch of spawn floors for the slimes and light them up so that nothing else spawns on them?

Best Answer

The Mob Spawning section of the Minecraft Wiki goes into some detail about what the requirements are for a mob to spawn. The parts relevant to this question are:

  • If it's not a squid, then...
    • the spawning block itself must be non-opaque and non-liquid
    • the block directly below it must be opaque or upside down slabs / stairs and may not be bedrock
    • the block directly above it must be non-opaque
    • the spawning mob must not be obstructed by solid blocks, other mobs or liquids

There are other conditions on Slimes, but those are concerned with them being in the right chunks, rather than the precise blocks. So yes, for every non-squid mob the game tries to spawn, the mob needs to spawn on top of a solid block, with the block they spawn in, and the next one up, being non-opaque non-liquid, and with the block they spawn in being air so they don't fail the collision check. They will not spawn in mid-air.

Recommended practice for spawning slimes is a wide, flat floor, in a chunk they can spawn in, below level 40,

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