Enchanted Cavern

The Enchanted Cavern is a rare structure, located just barely the bedrock layer. It is surrounded by a thick layer of Prismarine. When it first spawns it replaces all nearby iron ore with water, gravel with lapis lazuli, gold ore with diamond, and redstone ore with gold in that order. The Enchanted Cavern features a flat grass+flowery area and a small pond in the center, with a large lamp-like structure hanging above it. A small glass haven hangs from the lamp, with a dispenser at the bottom, chest at the top, and a short description of the cavern in the middle. The dispenser contains an Enchanted Shard, and 'The Chest; TL;DR'. The chest features a unique sacrifice mechanic.

Given its rarity and its ability to spawn in most biomes, the best way to find it is via the Enchanted Cavern Locator spell. The cavern stops spawning after day 12.

Chest Mechanics
This is more complicated than it has any right to be. ''For a shorter rundown on the mechanics, click here. TL;DR : Put random stuff you don't need in the chest and you might get some useful stuff back.''

At the start of every day, the chest in the center of the cavern disappears for a short period of time. Any items inside the chest at time of disappearance are considered up for sacrifice. Depending on what items are sacrificed, you may be granted an effect, or given certain rewards. However, you are not guaranteed a reward from most items. The list of items eligible to give rewards is as listed: ''Note #1: Only enchanted Iron/Diamond/Gold/Leather items have a chance to give enchantment rewards by default; any other enchanted item can only give rewards via the 'booster' items below. ''
 * Any enchanted item; See note.

If not enchanted: ''NOTE #2: If you are sacrificing multiple of the same item, do NOT stack them in the same item slot; they will be wasted. They must be put in different item slots.''
 * Gold Tools/Weapons/Armor, excluding shovels and hoes.
 * Diamond Tools/Weapons Armor, excluding shovels and hoes.
 * Egg
 * Blaze Powder
 * Golden Apple
 * Nether Star
 * Brewing Stand
 * Enchanting Table
 * Ghast Tear
 * Emerald Block
 * Diamond Block
 * Carved Pumpkin

Reagents/Booster items
Although they don't grant rewards directly, the following items count as 'reagents' or 'boosters'. When placed in the chest, they increase the chance that the other items in the chest will be 'saved', granting a reward:
 * Emerald, and Emerald Block*
 * Gold Ingot, and Gold Block
 * Iron Ingot, and Iron Block
 * Diamond, and Diamond Block

More details on the exact mechanics are below in the Reagents/Booster item mechanics/probability section.

Possible Loot
The table below shows the list of items, their chance to be 'saved'(grant a reward; this chance can be increased via the booster items, with the exception of the special carved pumpkin and shield), and their possible rewards: ''*When sacrificed, a golden apple has a chance to heal all players near the cavern for 2 hearts at the start of each day thereafter. The chances are 25% for one apple sacrificed, 50% for 2 apples sacrificed, and 100% for 3 apples sacrificed. Sacrificing more than 3 apples will have no effect, and each apple must be sacrificed on separate days.''

** ''The Special Carved Pumpkin has a fixed chance of 1/64, and the Special Shield a chance of 1/2. These chances cannot be increased by boosters.''

*** ''Diamond/Gold items give their materials based on the items: Sword, Axe, and Pickaxes always give 1 materials, with a 25% chance at 1 more material and a 25% chance for 2 more material. Helmets and boots always give 2 materials, with a 25% chance at 2 more material and a 25% chance for 4 more materials. Chestplates and leggings always give 3 materials, with a 25% chance for 4 more materials and a 25% chance for 8 more materials.''

Enchanted items
By default, only enchanted Iron/Leather/Diamond/Gold/Bows can give Enchantment rewards. Any other kinds of enchanted items may only give rewards via the booster items above. Items that are 'saved'(see the above table for chances) will always grant enchantment rewards.

If an item is chosen to give enchantment rewards, they can give both experience bottles and the enchantments they contained, barring certain enchantments that have little to no use. Most enchantments have a 100% chance to give rewards in this case; the exceptions are listed below: The amount of experience bottles given is the same regardless of enchantment. Items always give at least one bottle, with a 25% chance to give 2 extra bottles, a 25% chance to give 3 bottles, a 25% chance to give 5 bottles, and a 25% chance to give 7 bottles.

''Note: These chance rolls are all independent of each other; it's possible to succeed each 25% chance roll for a total of 18 bottles per item, or it's possible to fail each one and only get a single bottle. On average, you will receive 4.5 experience bottles per item.''

Reagents/Booster item mechanics/probability
When a booster item is put in the chest, it has a chance to 'save' an item that otherwise failed its roll to get saved. This can save items not normally given rolls to be 'saved' by the chest, such as chainmail armor. Multiple boosters can be used, but they MUST be in different item slots. Any items stacked in the same slot will be wasted. If a booster roll succeeds, it will save one of the other items in the chest which had previously failed their roll to be saved. The chances are as follows: