I'm stuck underground. How do I get back to the surface?

You can do a couple of things. If you have a pickaxe, then you can mine the block directly above you, then collect the stone and jump + place it below you. Do this repeatedly until you reach the surface.

If you don't have a pickaxe, it will take a little longer since you can't collect stone without one. Instead, just mine out blocks in a staircase fashion, using your hands.

Even in a completely unlit area, there is still a little bit of visibility. You can increase the visibility further by turning up the Brightness slider in the Video Settings. If it still isn't enough, you can turn up your monitor brightness, change the gamma/calibration, or turn out the lights around you.

Once you can vaguely see the world around you, start digging to the surface. Even if you do not have a pickaxe, any ordinary block can be broken by hand (slowly). The simplest safe way to ascend is a diagonal staircase:

#####   #
####   ##
###   ###
##   ####
#   #####

Dig out three blocks in front of you and upward, wait to see if there are any gravel or lava hazards, and proceed.

Or, since your question indicates you're willing to die, move ahead before you dig the topmost block, and hope something falls on you! Better yet, simply dig straight upward (using the blocks you collect to fill the space below you), which will get you either to the surface or your death faster.

For editing, WebChest is an online inventory editor; just upload your level.dat, edit, and download the edited file.

If you dont want to use a inventory editor, and you are in a big enough area*, kill the lights and wait for zombies to spawn.

*I dont know what a *big enough area*** is. I hear that zombies will not spawn within a certain radius of you (I only observe that they find you at the worst times).

** A big enough area is a space that is at least 24 blocks across according to VirtualBlackFox's comment.

You can take falling damage from falling just a few blocks. You'd have to dig out 10 blocks maximum with your hands to make a staircase that would let you fall far enough to suicide.

You shouldn't have to kill yourself when you go exploring. Just remember where you came from. Mark a path or something. Bring your stuff with you so you can function when you're away from you house/fort/castle thing. You could dig a stairwell just high enough that you will damage yourself by jumping from it and kill yourself that way if you really want to kill yourself.

If you're stuck underground and you can't see around you:

Turn up the brightness on your monitor or in your graphics card settings. It's kind of cheating, but it makes it easier to see your surroundings and where you might have come from.

Try to find the path you came in on:

If you are able to find this path and there are cliffs you are unable to climb, try punching stairs into the walls or use the spare stone you collected going down into that hole to make 9stairs to get back out.

If you are unable to find the path you came in on:

Then explore the caves trying to get to the highest point possible. From there punch out blocks above you filling in block below by jump placing them below you. This will of course be very slow but hopefully you've found a good high point which will leave you near the surface.

If all else fails:

/kill

To prevent this in the future:

At the very least bring a small stack of logs or wood. Since you're far underground with no pick you'll be able to make sticks and you will have stone in your inventory to make a pick. With logs you will also be able to make your own coal and more sticks to make torches. If you ever start to run low on wood head back to the surface, unload your shiny stuff and get more wood.

I would say turn it to easy, as more zombies/skeletons/spiders spawn on easy, though they won't do much damage, or you could just do what everybody else is saying to do. Not to mention, there has to be at least one water/lava spring block somewhere!

If you're not abhorrent to a little cheating, if only to get yourself back to the surface, you should try TooManyItems (by "Marglyph"). After you get it set it, if you press 'O' on your inventory screen, you'll toggle a graphical list of every material in Minecraft. Clicking on one will give a number of that material to your inventory.

If you ARE abhorrent to a little cheating, then this is still an excellent option. In addition to getting free stuff, you also get the options to Save up to 7 inventory configurations, and delete items permanently from your inventory.

So what I'd suggest, is that you save your near-empty inventory, give yourself enough material to get back home, and then reload your old inventory.

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To set it up, simply download it (18.01Kb), extract everything in the folder, and then copy the craftscripts folder, minecraft.jar, and WorldEdit.jar files directly into your bin folder for Minecraft. If you can't find the hidden file, the default is

C:UsersUserNameAppDataRoaming.minecraft (Replace "UserName" with your username). Note: You will likely have to enable the viewing of 'hidden files and folders' on your machine, as AppData is hidden.

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One word of warning: This mod is very quick/easy to set up, but it requires you to overwrite the minecraft.jar that is already in your bin folder. If you want to remove the mod afterward, all you need to do is copy the original minecraft.jar somewhere else, and then once you're done with the mod, delete the craftscripts folder, and WorldEdit.jar file, and replace the new minecraft.jar with the original one.

Updating the game will also remove the mod, as it will update your minecraft.jar file, but it won't remove the craftscripts folder or WorldEdit from your bin folder. However, leaving those two items there will not affect your game at all, so updating your game is a much faster way to remove the mod from your gameplay.

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REALLY IMPORTANT EDIT: With 1.5, this method of installation no longer works: you need to append the class files of the mod into the minecraft.jar file. If you're unsure how to do this, you can simply download a copy of the already modded minecraft.jar Here.

You're on a Mac, you say? Download Inside Job. Open it, choose your world, and give yourself a diamond pickaxe/sand or gravel/ any building material. Get out of the hole by building a staircase/suffocating yourself/putting blocks under you. If you're afraid of your friends laughing at you because you cheated, just delete it off your system.

MCEdit works on the Macintosh and would be a utility you can use to move your character (or at least carve out access to the surface.)

What I do is I put all of my torches to the left side of me. That way, when going back up to my base camp, all of the torches that lead up to the very top will be on my right hand side. Another good way is to bring a bunch of Redstone Torches. Every Time you reach the beginning of a new tunnel place a torch. In the future you will know that that way is not the way to the entrance of the cave.

I had this problem too, if anyone else finds themselves with this problem - mine stuff but make sure you mine upwards in a stair-like design.

The lucky part for me was that I was mining and found a cave so I had open space and I placed bricks down to jump on. If you have spare blocks use them. Use every tool you have. Just keep digging higher until you reach the surface.

Do not kill your self otherwise you will have to earn everything you had back!

If you're OK with using the console, which many consider cheating, you can use the command /gamemode sp.

This changes your mode to "spectator", where you can freely move around, avoiding collission.

In this mode, you are free to move back to the surface level.

If you have a shield, you can go next to a creeper and have it blow up, collecting the blocks it drops to be able to jump up.