Hardcore

Hardcore mode is a new game/difficulty mode that is set to be released in a future update. The mode will be set from the world generation page. Once you generate a world with hard-core mode, you can't go back. Hardcore mode has been implemented into the 1.9 pre-release 2

Hardcore mode deletes the world when the player dies, and locks the game to hard difficulty.

The idea came to Notch from a Minecraft gameplay blog on the PCGamer site.