Underground animal farm isn't working

Question

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?

Answer

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}

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