Hardcore

Hardcore mode is a new game mode that is tied to the eponymous difficulty and will be available in Beta 1.9. The mode will be set from the world generation page. Once you generate a world with hard-core mode, you can't go back.

When the player dies, hardcore mode only offers a button to delete the world save instead of survival mode’s or  button. The world save still exists until the button is pressed, leaving opportunity for one to keep the world if they really want to by using 3rd party tools. While starting a hardcore world, the game is set to “hardcore” difficulty, which only differs from “hard” difficulty in the fact that it cannot be changed to another difficulty while playing the world. Also, if the gamemode is set on hardcore, the picture of the hearts of the life gauge change. When the difficulty is set through the main menu's options screen, using peaceful will not spawn any hostile mobs with the exception of cave spiders.

The idea came to Notch from a Minecraft gameplay blog on the PCGamer site, almost a year prior.

Hardcore mode was first available to the players in the 1.9 pre-release 2, and officially implemented in 1.9.

Trivia

 * In the world selection screen, Hardcore mode is written in red.