Nether Reactor

The nether reactor was a player-built structure exclusive to which, when activated, a massive structure of netherrack spawned large quantities of normally rare or unobtainable items.

Creation
The reactor was constructed by placing gold blocks, cobblestone, and a nether reactor core in the 3×3×3 arrangement shown to the right. Air blocks were required on the middle and top layers. If the structure was incorrect, attempting to activate the reactor resulted in the client message "Not the correct pattern!".

Behavior
To activate a nether reactor, the following conditions needed to be met: If those conditions were met, tapping the core would activate the reactor, sending the client message "Active!". Immediately after activation, a massive structure of netherrack with multiple rooms, referred to as a Nether spire, was generated around the reactor. The bottom room, which contains the reactor, replaced all blocks other than those of the reactor with air, while blocks in the upper levels remained intact.
 * All players were in the range of the reactor's effects on activation.
 * The reactor was built within the valid range (Y: 4–96).
 * The player who activated the reactor was standing level with the structure.

Items would begin to spawn within the reactor room, including glowstone dust, nether quartz, cacti, sugar canes, both types of mushrooms, bows, bowls, books, doors, beds, pumpkin seeds, and melon seeds. Zombie pigmen were also able to spawn. During this stage, the blocks that made up the reactor were converted into glowing obsidian, and the core adopted a red texture to symbolize activation. If the player mined the active core, all aspects of the reaction froze.

The reactor cycle would end after 45 seconds, damaging the reactor's structure and changing the world to night. The reactor was replaced with a 3×3×3 box of obsidian containing the core, which adopted a darker, blue-black texture to indicate that the reaction has ended. If the player built a new reactor around the burnt-out core, the netherrack spire would generate, although the rest of the reaction would not take place.

Trivia

 * The spire was composed of over 25 stacks worth of netherrack.
 * Excluding the core, any part of the reactor could be mined after activation and it would still function. This way, the player could recycle the expensive gold blocks. It was possible to recover all 4 blocks provided the player worked quickly.
 * Because the spire could not replace blocks past the world border, when the reactor was activated near the edge of old worlds the player would be able to see the time set to night.
 * If the nether reactor core was mined out while the nether reactor is active, the glowing obsidian would not turn into obsidian after the reactor deactivates. This also caused the reactor to stop spawning items and zombie pigmen, and make the day/night cycle freeze at nighttime for several in-game days.