After finding this thread a few months back I've been a minecraft fanatic! Granted I haven't done any insane projects like I've seen posted on the net, but I have been having quite a time building myself a castle and trying to work out how to get an indoor tree farm going.
And i've also learned never to dig straight down...there's lava down there! Resulted in two deaths trying to get all my gear back!
Yes, never dig straight down. Dig horizontally or diagonally, or jump side to side, but not straight down!
Indoor tree farms are pretty easy. Make a room at least eight blocks tall (enough clearance for the trees to grow), and put a layer of dirt blocks in the floor. Then, arrange torches/trees in this pattern:
Go out and do something else, and when you come back there should be some trees grown. I usually just cut down the ones that have grown and leave the rest. I can typically clear the grown trees 3 or 4 times before I need to replant, since they don't all grow at once. You should easily pick up enough saplings to replant while you do this, too.