Dropped items will eventually despawn. The amount of time it takes differs, they go away more quickly if you move further away from them.
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go into the minecraft source file(windows=Run-%Appdata%-->.minecraft/mac=finder-->Username--> Library--> application support-->minecraft) then go to saves then your world and copy the level.dat file to the desktop. go to minecraft then put all items into your chest. save and exit, then quit minecraft.replace the new level.dat with the one on the desktop and there you go, duplicated items!
Snow can go away in minecrafts, and can melt very slowly but it will still melt in the category its in on or by the minecraft.
No, it doesn't make them unbreakable. the "Ubreaking" enchantment in Minecraft just increases that certain item's durability (How it it can go without breaking)
Going to the Nether you can get unique items only found in the nether and monsters. Items only found in Nether are: Netherrack, Soul Sand and Glowstone. The Mobs only in the Nether as of Minecraft V1.8 Beta are: Ghast and Zombie Pigman.
You must died somewhere else away from your spawn point. You just have to go back to find the dropped item.