Structure

Every specific group of blocks purposefully formed in Minecraft based on coding is part of a natural structure.

The Overworld
The Overworld contains numerous generated structures, at a wide variety of scales.

Terrain
Biomes dictate the shape and height of the world. At this stage, the ground is made entirely of stone, with water filling in most empty spaces below layer 63, with exception for structures.

Mountain
Mountains are hills with extreme slopes and cliffs. Mountains can sometimes have caves through them. On an amplified world, mountains are extremely common in all biomes except ocean and swamp biomes.

Floating "island"
Floating "islands" are structures that float in midair that are not connected to the ground, the sea, hills or cliffs. Floating "islands" are normally just random pieces of floating dirt and stone found near cliffs, but on rare occasions they can be large, floating structures that even have springs and trees on them. Floating Islands are most frequently found in the extreme hills, ice plains, ice mountains and mushroom island biomes, but can be found anywhere. Floating Islands are very common in an amplified world.

Hollows
Hollows are the opposite of floating islands. They look like caves, but they have nothing to do with cave generation (although they may intersect with them). When there are many overhangs, they close together and create a hollow. They have exactly the same floor as the terrain above, depending on the biome that they are located in, unlike caves. Hollows have no specific floor. Grass can generate inside too, and interestingly will survive without light. When they generate under the sea level, they are filled with water. They are extremely rare in the default world, but can be found in the customized world.

Hill
Hills are randomly generated pieces of land in the map. Like stairs, hills are always traversable to their lowest point by virtue of the algorithm which generates them; there is almost always a place on each level from where the next level can be accessed, meaning that the player can climb a hill one level at a time until they reach the top. Cases where this is not true are rare.

Beach
Beaches usually generate next to oceans and cover all nearby shorelines. They come in two different varieties: sand beach and stone beach. Sandstone is located below sand in sand beaches. Stone beaches are usually steep cliffs that lead straight into the water.

Surface layer
The uppermost layers of the terrain are converted to a biome-dependent material: usually grass blocks and dirt, or sand in deserts and beaches. Podzol is found in mega taiga, mycelium in mushroom island biomes, and red sand is found in the mesa biome. Sandstone is generated under sand.

Basin
Occasionally, instead of being converted to dirt or sand, the top layer is stripped away, leaving a 'basin' of bare stone. They bear some resemblance to a geological 'shield' (an area of tectonically stable rock that has been exposed to prolonged erosion due to its very old age; it is distinct from the geological term "basin"). They seem to be more common in forest or plains, and are occasionally seen filled with water. Commonly, minerals can be found in these, generally coal ore and iron ore. If generated in a Mesa biome in the PC version or the Pocket Edition, gold ore can also be seen.

Ocean
Oceans/Seas are huge bodies of water with every single water tile being a source block. The ground can rise high enough to produce small, relatively barren islands. Prior to the Adventure Update, oceans were generated as part of the terrain generation algorithm, but since then, they are part of the ocean biome.

Ocean monuments have a chance to generate within deep oceans.

River
Rivers are long "strands" of water that cut through or separate biomes. They do not have a current. Patches of sand and gravel are seen commonly dotting the banks of the rivers, and clay settled at the very bottom. In some cases, rivers will generate higher than normal, producing a dry riverbed, sometimes with occasional small water patches.

Lake
Lakes are small bodies of liquid. Water lakes, which are small pools of water springs, can generate above sea level or inside caverns. They can also generate isolated underground, connected to no other structures whatsoever. When in a winter biome, these small lakes are never initially frozen but will turn to ice if exposed. The lakes can also be composed of lava; however, lakes of lava are much rarer. Lava lakes found at the surface are surrounded by stone (which can be replaced by ore veins such as dirt, gravel and coal). Both types of lake generate with a small air pocket above them, which may result in floating sand, floating snow cover or even the top 2/3rds of trees above the lake. Lava lakes may cause trees to burn away.

Cavern
Caverns are caves and tunnels that are automatically generated under the ground in various places. Caverns are composed mainly of stone, and expose large amounts of ores and igneous stones. When caves are generated so their entrances are located in a mesa or desert biome, some of the sand around cave entrances can be found compressed into the biome's respective sandstone.

Ravine
Ravines are tall, long cracks of air, usually measuring around 30 to 50 blocks in height, 84 to 112 blocks long and no more than 7 blocks wide. Ravines can have small ledges along the top. They can be found at levels 20 to 68, sometimes appearing on the world surface or underwater, forming canyons and trenches. They can go very deep underground, sometimes spawning slimes or exposing diamond ore. If they reach deep enough, they may also be floored by the lava lakes at level 11. In the Amplified world type, ravines can reach from surface level to bedrock. Ravines can also be found in oceans or rivers having a waterfall from the river/ocean falling into them.

Ravines can connect to caves, dungeons, abandoned mineshafts, and any other generated structures. Due to the large surface area of their walls, ravines often have water and/or lava flowing down them (from springs in the walls, underground lakes, or even openings to the sea).

Ravines do exist in Pocket Edition beta test 1.2 build 1.

Abandoned mineshaft
Abandoned mineshafts are structures generated underground which consist of branching mining tunnels with wooden supports and broken rails passing through it. Cave spider spawners can be found in mineshafts. Open areas generate with oak wooden bridges instead of leaving floating platforms. They contain minecarts with chests that may have rare items such as diamonds and horse armor. They may also contain iron ingots, coal, pickaxes, and other useful items. They are likely to expose veins of coal, iron, lapis lazuli, gold ore, and rarely, emerald and diamond ore. Abandoned mineshafts will also generate partially above ground in mesa biomes.

Stronghold
Strongholds are structures that contain multiple rooms, doors and other aspects (some of these rooms include libraries, fountain rooms, dungeon-like rooms, etc.). There are 128 per world. Stone brick, mossy stone brick, cracked stone brick, doors, iron bars, monster eggs and stone slabs are the materials that make up the strongholds. They also contain silverfish, which one should be careful of when digging around in strongholds. Portals to The End are found in specific rooms in strongholds. Chests may be found containing items such as books, redstone, ender pearls, apples, bread, compasses, iron tools/armor, and diamonds. Strongholds can be found by right clicking an eye of ender, which will be thrown into the air and point the direction where the End portal is, showing the way to the stronghold.

In Pocket Edition, strongholds have a chance to generate under village wells.

Small structures
The quantity of most of these features (aside from dungeons, mineral veins, and springs) is biome-dependent; not all features can be found in every biome.

Dungeon
Dungeons are small, mostly underground, one-room spaces bordered by moss stone and cobblestone, and typically contain chests with rare items, and a hostile monster spawner in the center. Dungeons generate with either a zombie (50% chance), skeleton (25% chance), or spider (25% chance) spawner. Occasionally a player may find that the spawner is covered with gravel and will be harder to reach, yet the mobs can still spawn with the gravel around it. The chests may contain rare valuables, such as music discs, enchanted books, saddles, horse armor, and golden apples. Rarely, multiple dungeons can generate next to each other. Dungeons generated in a desert can sometimes be visible from the surface due to the sand falling down.

Mineral vein
A mineral vein is a natural deposit of ores. Players can come across these veins in caverns or anywhere where there is natural stone. Underground deposits of dirt and gravel are generated in this step, followed by the more precious ores: coal, iron, gold, redstone, diamond, emerald (in extreme hill biomes) and lapis lazuli. They can only form in stone, and do not replace each other or any other block. However, there is one exception: other ores can replace andesite, diorite and granite. Note that two or more mineral veins can form next to each other and make it look like a mineral vein made of more than one material.

Tree
Trees are common structures created both during world generation and by players (grown from saplings). They are made of wood and leaves, and in Pocket Edition might have vines or mushrooms (as dying trees and fallen trees respectively). Tree sizes range from small trees that can be fully harvested from ground level, to large trees that may require several minutes to harvest.

Huge mushroom
Huge mushrooms are structures that only naturally occur in mushroom island biomes, roofed forest biomes, and in Pocket Edition, swamp biomes. They are composed of mushroom stalks and either red or brown colored blocks for the cap. They can be grown outside mushroom islands by fertilizing red or brown mushroom with bone meal, or can be generated with mycelium.

Spring
Springs are randomly generated blocks of either lava or water that act as a source of their respective material. While both can be found on the vertical side of stone blocks above the surface, lava springs are more often found underground beneath layer 32 in caverns and mineshafts.

Desert well
These well-like structures built of sandstone blocks and slabs generate only in the desert biome. They have a 1/1000 chance to be generated in any desert chunk, which makes them a rare sight. It is possible for a well to generate around a cactus. The well structure will still generate with the "Generate Structures" option disabled. In Pocket Edition, it is common to find two desert wells spawning in the same chunk.

Moss stone boulder
These structures are meant to represent boulders, made entirely of moss stone. The arrangement of these structures varies greatly. They can be found dotted around areas of the Mega Taiga biome. Moss stone boulders are quite rare, due to the mega taiga biome's rarity.

Ice spike
Ice spikes are tall spires made of packed ice. The spikes can only be found in the ice plains spikes. There are two variants of ice spikes: one is short and thick, and the other is extremely tall and thin.

Fossil
Fossils represent the remains of giant extinct creatures, and are composed of bone blocks, with random blocks removed and some of them replaced with coal ore. They were made with NBT structure block format and they have NBT files of different types of fossils in minecraft.jar/assets/minecraft/structures/fossils. Fossils randomly generate 15-24 blocks below the surface in deserts and swamplands, including the "Hills" and "M" variants. Each chunk has a $1/64$ chance to generate a fossil. They may generate as one of four different variants of skull or one of four different variants of spine.

Buildings
Buildings are naturally generating structures that form above ground. They can contain valuable treasure, but traps and puzzles as well.

Village
Villages are generated in desert, plains, taiga, and savanna biomes, and are a site for villagers, with whom the player can trade. They are composed of a random selection of various buildings and farms. The village will always generate with one well, along with other buildings that can be any of several different houses, single or double farms, blacksmith's shop, library, butcher's shop, or church. Villages are built with different materials depending on their biome; villages in plains biomes are comprised of oak wood, oak planks and cobblestone, and villages in deserts are built with varying types of sandstone. Taiga villages are made of spruce and savanna villages are made of acacia. Villages may generate partly submerged in water. This results in some of the path being made of wood. The villagers move about in the area around the village, retreating indoors at night and during storms. Villages have three-block wide paths to join all houses.

Desert temple
Desert temples, also known as pyramids, are found in deserts. They mainly consist of sandstone and stained terracotta. In the middle of the temple is a block of blue terracotta, which if destroyed, reveals a deep, hidden chamber underneath, where players can find 4 chests, which can contain valuable loot, and a stone pressure plate trap connected to a 3×3 grid of TNT. Treasures found in the chests include emeralds, diamonds, iron ingots, gold ingots, rotten flesh, enchanted books, and bones. While rotten flesh, bones and gold ingots are found frequently, other treasures are not as common.

Jungle temple
Jungle temples are found in jungles, and mainly consist of cobblestone and mossy cobblestone. There are three floors, the bottom floor containing a chest and a puzzle. The chest is found through a corridor which contains hidden tripwires that triggers a dispenser to shoot arrows at the player. The player must carefully navigate through the corridor while triggering the tripwires. The puzzle is located at the opposite of the corridor and consists of three levers that must be pulled in the correct order to open a secret door which reveals another chest.

Witch hut
Witch huts generate in swamp biomes. Among all hostile mobs, only witches spawn in the 7×9×7 area inside and around the hut, and a witch spawns in each hut during world generation. There is one empty cauldron, one crafting table and one flower pot inside each hut. In Pocket Edition, there is a red mushroom in the flower pot, and the cauldron is filled with one random potion or splash potion.

Ocean monument
Ocean monuments are underwater structures that generate in deep oceans. They generate in the shape of a pyramid with many randomly-generated rooms. They are composed of all three variants of prismarine and lit by sea lanterns. Eight gold blocks encased in dark prismarine can be found inside as treasures. Also, a random assortment of sponges may generate inside. Guardians and elder guardians spawn here.

Igloo
Igloos are structures that generate in snowy biomes. They were made with NBT structure block format and they have 3 NBT files of 3 parts of the structure in minecraft.jar/assets/minecraft/structures/igloo. The structure consists of the igloo house, as well as a basement sometimes. In the basement there is a villager and a zombie villager, both priests, locked up underground. The basement also contains brewing paraphernalia, with items that the player can use to cure the zombie villager. The basement is accessible by removing one of the carpets to reveal a hidden trapdoor.

Woodland mansion
Woodland mansions are massive house structures that generate in roofed forests. Inside the mansion is a wide variety of different rooms, the exact layout varying from mansion to mansion. Woodland mansions are the only place where evokers, vindicators, and vexes spawn naturally (but only once). The mansion can be located with a woodland explorer map.

The Nether
The Nether, though equally vast, contains far fewer types of generated structures than the Overworld.

Lava sea
Lava seas are found at and below level 30 in the Nether. They make a large portion of the Nether, and are extremely common. They can stretch for hundreds of meters in any direction, and are usually bordered by netherrack (or more rarely soul sand). Even on far render distance, the player is rarely able to see the other side of the seas due to their vastness.

Nether fortress
Nether fortresses are very large complexes made mainly of nether bricks (with added nether brick fences and stairs to be found). Nether fortresses can often tunnel through netherrack, and the inside of the tunnels will be cleared of netherrack by the terrain generator. Blaze spawners and nether wart farms are only found in nether fortresses. Blazes, zombie pigmen and wither skeletons can spawn anywhere in the fortress. Nether fortresses have loot chests generated in indoor corners.

Glowstone clusters
Glowstone clusters are typically veins of glowstone that can be among the hardest natural materials to harvest that don't require digging. They form in coral-like structures on the underside of hanging Netherrack, so they are often found on the ceilings of the Nether, where they provide light along with the ever present lava.

Glowstone is renewable in the Console Edition of Minecraft, as there's an option to reset the Nether in the world options.

Other things located in the Nether
There are areas of soul sand and gravel around layer 64, veins of nether quartz ore, large "veins" of magma blocks and "hidden lava", which is a single block of lava generated randomly between netherrack.

The End
The End is the final and most barren dimension, with very few types of significant structures, until after defeating the Ender Dragon, which will open gateways to more structures.

The Central Island
The center of the End is a large, asteroid-like island composed entirely of end stone, floating in the void. At a distance of 1000 blocks away, an endless expanse of more islands begins, away from the main island. These consist of large islands, about the size of the main island, and smaller ones, which are usually very thin and small.

Obsidian pillar
Obsidian pillars, also known as obsidian spikes or obsidian towers, are tall, tower-like structures that generate in The End. Y level of pillars are 76, 79, 82, 85, 88, 91, 94, 97, 100, and 103. Also, bottom of pillars' Y level is 0, means that pillars are generated through the island. Ender crystals will spawn on top of each one to heal the Ender Dragon.

10 pillars are generated in a circle around the fountain. Ender crystals of second and third shortest pillars are surrounded by iron bars, which must be destroyed before the crystal can be taken out.

Obsidian platform
The obsidian platform is a 5 by 5 square of obsidian that is generated once a player enters the End. (Note that if the obsidian is destroyed, or if a block is placed on top of it, when the player enters the End, the obsidian will be restored and any blocks on top of it will despawn.) Obsidian platforms generate at X, Y, Z = 100(100.5), 48, 0(0.5), mostly far away from the island, making it tough to get there. Players who enter the end will spawn at X, Y, Z = 100, 49, 0 and the other entities that enter the end will spawn at X, Y, Z = 100.5, 50, 0.5, the middle of platform and 1 block higher than the platform.


 * Sometimes the platform generates inside a case of end stone. It will remove enough end stone so that the player can walk around.
 * Occasionally the platform will generate in midair.

Exit portal
When the player first comes to the End, an exit portal in the shape of a fountain is found generated in the middle of the pillars. The Ender Dragon will fly to the fountain from time to time, and spew Dragon Breath (Ender Acid in the Console Edition). Additionally, in the Console Edition, the dragon will defend the fountain from endermen every 3 minutes or so. Upon defeating the Ender Dragon for the first time, the Dragon Egg will be set atop it like a pedestal, and the portal back to the overworld will activate.

After entering the fountain, the Minecraft credits come on the screen, implying that you have "beaten the game" (despite the fact that the game never ends). You can skip these credits by pressing. After the credits have finished (or after you skipped them), the player returns to the last bed they slept in (or their original spawn spot if the bed was obstructed).

The fountain is made of bedrock, making it indestructible. Also, this fountain includes 16 hidden end stones to proof empty spaces.

End gateway portal
The end gateway portal is a portal to the outer islands of the End. It is a fast way to get there without bridging over or flying, and is generated after you kill the Ender Dragon. You can get a new one with each new dragon you kill, with up to 20 created. It consists of the end gateway and two bulbs of bedrock. It is only accessible through ender pearls, breaking the bedrock in creative mode, or by gliding into it with elytra.

End city
End cities are structures that are naturally generated in the outer islands of the End. They were made with NBT structure block format and they have NBT files of different parts of the structure in minecraft.jar/assets/minecraft/structures/endcity. They are tall castle or tree-like structures constructed primarily of end stone bricks and purpur blocks and their variants. They are lit up by End rods. They are also the only places where shulkers can be found.

End ship
End ships have a fair chance of generating outside of end cities. They were made with NBT structure block format and they are a part of end city structures and the NBT file is located in minecraft.jar/assets/minecraft/structures/endcity and the file is called ship.nbt. Although difficult to reach, they are filled with extremely valuable loot, and are the only place where elytra and a dragon head can be legitimately obtained.

Chorus tree
Chorus trees generate on the outer islands of the end. They are formed in tall, coral-like patterns.They consist of chorus stems and chorus flowers. A chorus flower will generate at the top of each branch. They are the only source of chorus fruit and chorus flowers. You can grow a new chorus tree by planting a flower on a block of endstone.

Return portal
These portals generate randomly throughout the outer End islands. They appear similar to normal End gateways, but will teleport the player back to the center island's spawn platform, providing a quick method of escaping the End if one becomes lost.

Technical details
Structures are generated for a given chunk after the terrain has been formed. The chunk format includes a tag called TerrainPopulated that indicates whether structures whose point of origin is in that chunk have been generated. If it is false or missing, they will be generated again. Structure generation is based on what is already in the chunk, so (for example) flagging a chunk that has already been populated for repopulation will approximately double the amount of ore in it.

When structures are generated, they can spill over into neighbouring chunks that have been previously generated. Thus, a tree at the edge of the generated world (and probably only visible using external tools) may be overwritten by a lake before you reach it. It is also theoretically possible for two worlds generated with the same seed, from the same version of Minecraft, to differ slightly depending on the players' travel routes, because the order in which chunks are generated may determine which of two conflicting structures will overwrite or suppress the other.

Trivia

 * In previous versions, before snow cover was solid, a lava lake with floating snow cover above it could be a deadly trap before the snow melted.
 * The smallest possible fully-grown chorus tree (assuming the growth is not obstructed) would have 5 chorus plants.
 * Internally, witch huts, jungle temples, desert temples, and igloos are all the same structure, but they appear different depending on the biome. This means that they cannot be distinguished when using.