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. Also, if the gamemode is set on hardcore, the picture of the hearts of the life gauge change.

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