No
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Yes there is such thing as the Ether World in Minecraft.
Not that I have seen so far but the world is infinite so there could be
An underwater world seed for Minecraft is used to open an underwater world to play the game in.
Sorry it's probaly not possible ):
Yes, Minecraft is global.
Well you could either Burn it with Flint and Steel or you could blow it up with TNT or you could just leave it there and start a new world. Also you could delete the whole world.
No
Place the world in your minecraft bin's saves folder.
I don't recall any epic minecraft saves, but a quick look around the forums could point you in the right direction. As for world seeds, gargamel (exactly how it is written, including lowercase) is a great seed if you want to start a world from scratch.
It is not a seed, it is a world download. Look up dropper world download on google or minecraft forums
Any world on Peaceful........................