It is the nickname for bedrock, which is the peppery coloured block; which looks like stone, only of a darker contrast, and it is found at the very bottom of the overworld, the top and bottom of the netherworld, and in structures in the enderworld, since there is no top or bottom in the enderworld, it cannot be broken, and if you dig too deep, you will fall into the void and die, with the case of the enderworld, there is no bedrock to catch you, so you will fall into the void and die anyway, so be careful my friend!
No creepers can't destroy obsidian blocks but they can destroy a working portal.
Slimes still spawn, but only within 15 blocks above bedrock (adminium) and in large cavernous areas. If you dig down really far, and find a nice big area within 15 blocks of the bottom, you might find slimes. Another thing about slimes, is they spawn on any difficulty, including peaceful, but only the small one's will spawn on peaceful. There's a glitch where big slimes don't de-spawn when you change the difficulty.
It is around 60 to 64 blocks down as it isn't flat. Some bedrock pieces will be around 60 and some will go down to about 64. People say it is supposed to be the nether under bedrock as if you climb up in the nether you will find bedrock just like the floor in the overworld (Real minecraft world) I have seen what is under bedrock and it is just plain nothingness. You will see stars and if you fall out you will die and the game will crash maybe. I hear you ask how did you find this out? Well I used creative mode and broke through. I used to break it with Single player commands mod with the super pick-axe command.
Unless you're using a mod that specifically changes mineral distribution, the only thing that changes ore distribution is elevation on the map. So you understand the coordinates - full numbers are the boundaries between blocks. Level 0 is the top of the void underneath the world (which is no longer accessible, as far as I know), and the top is...I guess it's 255 or 256 now. (It used to be 128.) Some useful reference measurements: Bedrock/Adminium only shows up in 0-5. Lava tends to show up in vast quantities below 10. (Thus I recommend not mining below 10, as you might suddenly find your drift ending in a wall of lava - and if you're anything like me, you'll be watching a movie or something as you drift-mine.) Sea level is at 64. The ore-spawning levels I have memorized are from before the sky limit increase. Excepting coal, I think everything is still the same (after all, sea level didn't change and everything but coal had an occurrence elevation based on sea level), but be aware that some of these numbers might be a little off. I do know diamond is the same, and it's the one you should be using to define the upper limits of your mine. Coal: Probably off the top of the map. I see no reason they'd have changed it - it was 128 before the height change, so it's probably 256 now. Iron: 64 and below. Anywhere below sea level Gold: 32 and below. The lower half of the underground Diamond and Redstone: 16 and below. The lowest quarter of the underground Lapis Lazuli: 10-18 or so. It's the only one with a lower limit above void, making it a bit of an oddity. The best area for mining in terms of safety and material yield is between 10 and 16. That should get you out of most of the lava while leaving every block you mine capable of being any type of ore. The following mining pattern should enable you to directly see every block in that area. x is a mined block, o is one left behind: xxooxxoo oxoooxoo oooxooox oooxooox oxoooxoo xxooxxoo As you can see, every unmined block (o) is exposed to a mined block. The top and bottom levels have a shelf carved out so you can see one more strip of blocks - that which would, in an infinite pattern, be exposed by the floor of the next level up and the roof of the next level down. (You might have to take the block above the 1 block gap in the lower level, too, so you can see the extra block you exposed. This is an optional extra - the odds of missing out on anything significant by just cutting 1x2 tunnels are quite low. If you meant materials instead of minerals, most rare substances (including coal, iron and gold, actually) can be more efficiently produced in sustainable generation structures of some kind or other. These vary in complexity from chicken coops for egg farming to waterless mob grinders for gold. Clay's the only thing you really have to look hard to find that you can't later reproduce, except for ores.