Huge mushroom

The Huge Mushroom is a naturally occurring structure which was added to the game in Beta version 1.8. Huge mushrooms are composed of several blocks: mushroom stalks and either red or brown colored blocks for the cap. These blocks provide a valid spawn area for hostile mobs, and may be destroyed by hand to yield 0-2 normal mushrooms. The texture for the blocks which make up the cap depends on their position on it; for example, the corner block of the brown variety has 3 sides with the brown texture and 3 sides with the pore texture, while a block in the middle of the cap has only its top face covered with the brown texture and the others covered with the pore texture.

As of Minecraft 1.0, huge mushrooms occur naturally in the mushroom biome.

Growth
Huge mushrooms can be grown by applying bone meal to a small red or brown mushroom that is planted on dirt, grass, or mycelium; this works in both the Overworld and the Nether. The axe is the best tool for destroying huge mushrooms. When destroyed, large mushroom caps and stalks have a chance of dropping small mushrooms of the same type they were grown from. Each giant mushroom can be expected to drop 2-25 small mushrooms, making this a viable source of food.

A huge mushroom, brown or red, may grow to be 5, 6, or 7 blocks tall; no taller or shorter. They require a 7×7×6 to 7×7×8 space to grow depending on their height. However, the bottom-most layer (where the mushroom is placed) only requires blocks immediately adjacent to the source mushroom to be free, not counting diagonal blocks.

Whenever bone meal is used on a mushroom, it randomly selects a size to attempt to grow. The maximum height that will succeed is one less than the amount of free vertical rows available above the mushroom; thus, if a block is placed 7 meters above the mushroom, anywhere in the 7x7 row, it can only grow to be 5 blocks tall. If the game selects it to be 6 or 7 meters tall, the bone meal will fail on that attempt, similar to how it behaves with tree saplings. This allows the player to force huge brown mushrooms to always be 5 blocks tall, but may consume significantly more bone meal due to failures.

A huge mushroom will not grow above the height limit/skybox without modifying the game.

Huge brown mushrooms
Huge brown mushrooms consist of a single stalk in the center, with a 7×7 canopy of brown mushroom blocks at the top with the corners missing.

Due to the space requirement matching the size of the canopy at the top, it is possible for a huge brown mushroom to grow with its canopy directly touching one next door. This gives explorers the ability to quickly assemble platforms out of huge brown mushrooms, which affords a sleeping platform safe from mobs when illuminated (a single torch in the center is sufficient).

Huge red mushrooms
Huge red mushrooms, like their brown counterparts, have a single stalk in the center, but a different canopy, composed of five 3×3 slabs of red mushroom blocks arranged above and around the stalk, forming a 'dome'. Even though the space requirement is the same, the actual huge red mushroom only ends up being 5×5 meters in area, so subsequent ones can be grown closer together than brown ones.

Due to their shape, huge red mushrooms can be used to build a basic hut, either by filling in the space between the cap and the ground, using pistons to push the cap of the mushroom to the ground, or by digging a large enough hole to accommodate a huge mushroom so that the cap intersects with the ground (may take multiple attempts; see above). Due to the height mechanics, one can always force them to be 5 meters tall with a block 7 meters above, so one can simply dig a hole in the ground, place a red mushroom, and dig out the 4 adjacent holes. While this may also take multiple attempts, the resulting huge red mushroom will be guaranteed to properly align with the ground.

Trivia

 * Huge mushroom blocks can be acquired in Survival Mode if they are mined with the Silk Touch enchantment. Blocks mined in this way can be used in a furnace, burning as long as wooden planks. The spore texture (right) appears on all sides of their blocks, as this is the default data value for the block, similar to how white wool works. There currently is no way to legitimately obtain mushroom cap blocks with the other specific data values.
 * In SMP, by contrast, blocks acquired in this manner have the texture of the block that was harvested. However, such a block will have spore texture on some of the faces that were concealed by other large mushroom blocks.
 * Mushroom caps and stalks cannot be spawned from the inventory in Creative Mode, nor can they be harvested through Enchanting due to the way blocks are destroyed in Creative.
 * However, through third-party editing, it is possible to harvest the stalks/caps in Survival Mode, switch to Creative, and spawn an unlimited amount of either block as normal.

History
In a tweet, Notch showed a picture of a 1.7 change-list (back then 1.8 was planned to be 1.7). Although it was completely blurred out and was at first thought of as a joke, Notch later stated that one of the pictures with the new lighting system and the change list had a secret in them. People all around the web started speculating.

One place where people discussed it was on the Minecraft Forums, where it was discovered that the tabs at the top of the change list, which were partly covered, could be decoded based on the one pixel tall pattern available in the image. After a user named "tmcaffine" successfully decoded the image, the tabs read: ExperienceOrb.java, changelist.txt, Level.java, Tile.java, HugeMushroomTile.jav(a?), HugeMushroomFeature.(java?), RandomLevelSource(cut). This led people to believe that huge mushrooms would be added, and indeed, later on Notch revealed a screen shot on Google+ in which two huge mushrooms could be seen as well as an NPC Village.

In the 1.8 pre-release, huge mushrooms did not generate naturally, but could be planted by the player by using Bone Meal on a red or brown Mushroom planted on dirt. In the pre-release version 1 of 1.8's Creative Mode, the blocks composing a huge mushroom and bonemeal were not included in the item selection menu. The only way to create huge mushrooms in this version of Minecraft is to give yourself regular Mushrooms and a bone, craft the bone into Bonemeal, then plant the mushroom and finally use the bonemeal to grow a Giant Mushroom.

Up to release 1.2.3, different mushroom cap types could be created in survival by using the block damage changer exploit with any huge mushroom block as the default block. This was patched in version 1.2.4.