I've built a mob spawning trap but mobs are not spawning.

It's a tower that is 10 blocks long x 5 blocks deep x 35 blocks high. At the top of the tower (33 blocks) is a spawn floor. The spawn floor is 10 blocks long x 3 blocks wide. There are signs on the edges of the spawn floor.

I've used a mapper to find all the caves in the immediate area and they are all lit.

When I'm standing on the ground (35 blks below the spawn floor) nothing will spawn.

The game is set on normal, single player, and mobs will at night on the ground, just never in my trap.

Here is a pic of spawn floor (torch added for pic, it is completely dark normally)

Spawn Floor

Here is a pic of the whole structure (tower is built in the cliff).

enter image description here

I'm not sure what I am doing wrong. Please help.

  • i'm not too sure but i think it might have something to do with the small width of your spawn floor.
    – chandsie
    Commented Jun 23, 2011 at 0:58

5 Answers 5


There are two points to remember that may seem contradictory: first, like Nicholas pointed out, you need to makes sure that your mobspawner is the only nonlit space in a pretty big area around you (I think the area's actually about 150x150 blocks), but second, you need a lot of empty space around. In other words, minecraft will choose places that have a lot of surface area (not volume) to spawn mobs in, so if you were to have a lot of open caves beneath your mobspawner, the game will flag that area as more likely to spawn mobs in. You just also have to make sure that it's all well-lit, then you've effectively tricked the game into flagging that area to spawn mobs in, but then conveniently your mobspawner is the only dark area where it can do so.

I would recommend you take a look at this tutorial. You may not want that particular style of mobspawner, but it goes over the prerequisites of any good mobspawner quite nicely.

  • Thanks for the answer and link. Watched some of it this morning, the thing he built is massive. I was just trying to build something simple.....guess I have some work to do.
    – bluesixty
    Commented Jun 23, 2011 at 14:40
  • It's quite worth it if you go that big; keirshar has measured some of his epic traps to output over 2300 items per hour. He has an easier style here link but the important thing to remember is that even if the mobspawner itself isn't that big, the area cleared should be, more or less. Doing it over water means saving torches, at the very least, and the frame is just for reference, so you can see what area the mobs will be spawning in, roughly.
    – Ketura
    Commented Jun 23, 2011 at 16:13
  • For a small simple mob farm I would recommend checking out EthosLab on you tube. They are nice for when you just want some items while you are working in an area alot as opposed to a huge area designed to dump out thousands at a time.
    – James
    Commented Jun 23, 2011 at 17:37
  • I added 5 more spawn floors of the same size for a total area of 180 square blocks. Mobs are spawning now, but only a couple. Mapped out a 150 blk square around the trap and was really surprised at how big that is. Guess i'll be doing some more spelunking to light up the caves I didn't include :)
    – bluesixty
    Commented Jun 24, 2011 at 16:09

I think the main problem here would be other places where mobs can spawn. Yes, you said you lit all the caves in the "immediate area", but about how large is that area? If I recall correctly, mobs can spawn in a 128x128 size box around the player. Most of these will immediately despawn due to being too far away from you, but it will still take away from the spawning rate inside the trap, which doesn't have that large of a spawning area. I recall from experience having a 30x30 sized mob trap (Which has a spawning area 30 times bigger) with a fairly low spawning rate for mobs. Your best bet is probably to enlarge the spawn area a LOT. With such a small spawn floor, even if you missed just a single cave, that would take out a HUGE (Probably over 90%) chunk of your mob spawns.

By the way, don't worry if even after enlarging the spawn floor and lighting up all the caves your mob trap doesn't work at night. Unless you've lit up a TON of area (A 128x128 area is equal to 16384 squares, and at a generous estimate of 64 squares per torch, you'd still need 256 torches to light it all up) around the trap, a lot more mobs will spawn outside of it than in the trap, and there's not much you can do about it except use a bed.

  • Great food for thought. I think I will add a bunch more spawn floors and see what happens. And recheck all the caves. My world is just riddled with them.
    – bluesixty
    Commented Jun 23, 2011 at 14:38

At night, mobs can spawn anywhere some will go into your spawning platform. When its daytime, some mobs will spawn on your platform but mostly they will spawn in caves that are not lit up, so torch up or completely fill in (not just block off) every cave near the platform and it should work fine.


You have to make 10 by 10 and at least 5 blocks high. That will be good enough now make the the killing floor at least 30 blocks deep.

  • 1
    5 blocks high? Why 5 blocks high? (Mobs only need 2 blocks of air to spawn, except for Endermen.) Commented Mar 21, 2013 at 2:27

You want grass to be there. Not stone or any other blocks. Not even dirt. Well dirt with grass =D

  • 1
    This is for spawning hostile mobs, not peaceful ones.
    – user28379
    Commented Mar 14, 2013 at 22:54
  • @jeffreylin_ The question doesn't mention peaceful mobs. Commented Mar 15, 2013 at 0:11
  • @MatthewRead Why would you have a kill floor for peaceful mobs?
    – user28379
    Commented Mar 15, 2013 at 0:15
  • @MatthewRead Yes it does, since it mentions lighting the caves. That makes it a hostile mob trap. Commented Mar 15, 2013 at 0:17

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