Minecraft wiki entry for Hardcore, associated text: When Notch announced Hardcore mode on Twitter, he jokingly posted an image of an 'ultra hardcore' mode where the premium status of the player's account is revoked on death.[2] This is not to be confused with the custom gamemode of Ultra Hardcore, which was invented later.
When a Hardcore mode world is loaded on a version released prior to Hardcore mode, the world is converted into a Survival world. Loading this world in a version featuring Hardcore mode does not revert the world back to Hardcore mode.
A popular challenge used by servers is a map in which player(s) must survive for 100 days on Hardcore mode while completing certain tasks. These maps may have custom features.