How to deal with magma cubes in the Nether?

I have created a large zombie pigman gold farm in the Nether.

But, the magma cubes are killing production. If I leave for more than a day, I get zero production, because there are magma cubes all buried under the lava. I have a plan to deploy cactus judiciously around their drop off points and that will keep them from congregating under the lava.

However, I'd like to build a rail station at the base of the farm, and I cannot figure out how to keep the cubes from spawning around / on the train tracks. I was hoping to avoid having to cactus the track for the entire route, as that would require massive amounts of sand and cacti. I was hoping for a more ... efficient solution. Perhaps to both problems?

1 Answer

There's an oddity in monster spawning you can exploit. As is well known, mobs can spawn on top of any block with a flat, opaque top surface, even if there is a non-solid block such as a torch or rail on top of that block.

However, the selection of center points for groups (“packs” as Minecraft Wiki describes them) of mobs is slightly different: it requires that there be empty space which is an air block, nothing else (not even a torch). Only once such a point has been found will mobs spawn with their feet at that level in the area around that point.

Therefore you can prevent spawning entirely as follows: make sure that in the vicinity (up to 20 blocks horizontally) of your rail station, at any particular y-level, either there are no empty air blocks, or at the y-level below it there are no regular valid spawning surfaces.

For example, consider this cross-section of an underground rail tunnel.

5 ######### _ slab
4 ### ### # stone/netherrack
3 ### ### R rail
2 ###R_R###
1 #########

No mobs can be spawned in the middle of a tunnel of this form:

• No mobs will spawn with their feet at levels 4 or 3 because there are no solid blocks below those levels (all individual spawn locations are invalid).
• No mobs will spawn at level 2 because there are no air blocks at that level (all pack spawn locations are invalid).

However, if there are any air blocks at level 2 within 20 blocks of the tunnel, then they may cause spawning. So, if the tunnel is in midair as such things often are in the Nether, you'd have to build a 20-block-wide flange beside the tunnel at level 2 to prevent spawns (preferably made of slabs so as not to itself be a spawning platform).

Disclaimer: I've never tried to engineer spawns in this way; I'm just going by others' documentation of the way it works. (And yes, this was already mentioned in the comments of this question; but answers should be posted as answers, and I felt a fuller explanation would be useful.)