How do I build an efficient underground farm?

Question

I prefer spending most of time in Minecraft hidden underground. I would like to build an underground wheat farm that fits the following criteria:

• Can be built entirely in a hollowed-out, underground room
• All of the crops have enough light and water that they can grow as fast as possible
• It is impossible -- or at least excessively difficult -- to accidentally destroy part of it (eg. knocking down torches)
• Mobs cannot spawn anywhere in it
• It is possible to continuously walk through it, holding down one of the mouse buttons and quickly chop down all of the crops and replant them (not at the same time, of course)
• Bonus points if there is any sort of mechanical means of collecting the wheat!

How would I build an underground farm that met this criteria?

Answer 1

I have the following underground farm. It's not optimal in terms of space, but for a farm, it's very efficient, and has automatic harvesting. Planting is always manual on these things unfortunately, and that's the bottleneck for massive production.

TOP VIEW

SSSSSSS
DDDDDDD
DDDDDDD
DDDDDDD
DDDDDDD
SXSSSSX

I replicate the above pattern, one level up to the right for as long as you want. S is stone. D is tilled dirt. The trick is with the X blocks. They are water blocks at the level of the dirt blocks covered with Stone.

LEFT VIEW

S    S
SDDDDW

When you view this from the left, you see there's a channel for water to flow out of your monitor. That is what permits automatic harvesting. The water blocks are necessary for hydrating the soil.

On the far right, I place a water gate.