Underground animal farm isn't working

I and some friends, we have a Minecraft SMP Server. We use the latest recommended Version of the Bukkit Server.

We dug a 40x40, 3 tiles high cave and placed grass at the bottom. Than we dug water canals and made a lava trap. The problem is that no mobs spawn there. Above the cave there is sand, but I think that doesn't really matter. The fact is that in other places on our server, mobs spawn. Does anyone know why they don't spawn in the cave?

Solution 1:

Actually, the problem is that the cave has a "ceiling".

[Animals] will always spawn on the highest available block in a column i.e. the one that can see the sky.
_{– Animal Spawning at Minecraft Wiki}

Solution 2:

Ok,

Yesterday I found a great soultion for my problem: The Mobiile Plugin for Bukkit. It has many options to set your own spawn rules, so that animals can also spawn in the animal farm.

Also it's great for creating a mob filled survival map on the server, just set all mobs to AGGRESIVE and set their spawn rate to 10 percent. =D

I just posted that, because eventually someone could need this informations too.

Leo