Hardcore

Hardcore mode is a Minecraft Game Mode that is a variant of Survival, differing primarily by the lack of the ability to respawn (instead, the world is deleted) and change difficulty. The mode was introduced in Beta 1.9 Pre-Release 2 and officially added in Minecraft 1.0. The idea came to Notch from a Minecraft gameplay blog on the PC Gamer site.

Features
While playing hardcore mode, 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

When the player dies, the game over screen only offers a button to delete the world save instead of survival mode’s and  buttons. The world save still exists until the button is pressed, leaving the opportunity for one to keep the world by using 3rd party tools. Exiting the client and loading the world again only reloads the game at the game over screen.

As of 12w18a, Hardcore Mode has been made as a difficulty for multiplayer, and dying while in the server does not give you a respawn button. Instead, it tells you to "Leave Server", and when you do it, you get the message "You have died. Game over, man, it's game over!". Try to reconnect to the server, and it just says "You are banned from this server!".

Trivia

 * While playing hardcore mode, the heart icons of the life gauge are different, as they show two small blood vessels (or eyes).
 * In the world selection screen, Hardcore mode is written in red.
 * When a world in hardcore mode is created with the Beta 1.9 pre-release, then played with the Beta 1.8 version, the world is permanently converted into a standard survival world.
 * When Notch announced Hardcore mode in Twitter, he also posted an image of an "Ultra Hardcore" mode where the premium status of the player's user account is revoked on death. This turned out to be a joke.
 * It is actually possible to respawn in hardcore mode, via defeating the Ender Dragon and activating the credits, after which you will promptly respawn, without world deletion or death.
 * Many Minecraft enthusiasts played Hardcore mode before the official version was released. It was simply on the honor system to delete your world if you died. This was often called an "Ironman" game.
 * Via hacking, it is possible to play a cross between Hardcore and Creative mode. The player can fly and place infinite blocks like in Creative mode, but the world will be deleted upon death. This is because there are separate variables determining if Creative or Hardcore is on.
 * If the obsidian blocks of a portal are lost (for example: thrown in the lava) or if the portal gets deactivated and the player doesn't have a Flint and Steel, then it's possible to get stuck in the Nether on Hardcore. The only way back would be to cheat the player some Obsidian or Flint and Steel or to change the player's location via MCEdit. If the obsidian for a portal isn't lost, the player still can trick a Ghast into reactivating the portal by igniting it with a fireball.
 *  It is possible to retrieve your world if you die during hardcore mode as long as you have not hit the "Delete World" button.  You can do this by going in to your "Saves" folder in Minecraft by going to Library>App Support on Mac, %appdata% on Windows or ~/.minecraft on Linux.

Bugs

 * In 1.9 prerelease 2, the "World Is Deleted Upon Death" was present regardless of whether the game was hardcore or not. This was fixed in 1.9 prerelease 4.
 * The "Leave Server" button you get when you die in HMP says "deathScreen.LeaveServer".