Biomes

Biomes refer to the different types of areas that a Terraria world can contain. Biomes can each contain their own characteristic terrain blocks, collectible items, backdrops, background walls, enemies, critters, theme music, Angler Quest fish, and other traits.

The most common biomes are Forests, located on the surface. Going farther left or right will eventually lead to The Corruption/The Crimson, Deserts, Snow, Jungle, the Dungeon, and at the far right and left of the world, the two Oceans.

Space spans the entire width of the world above the surface. Below the surface, there are several underground biomes: They begin at the Cavern layer and include the Underground Glowing Mushroom biome, the Underground Jungle biome, the Ice biome, and the Underground Desert. The very bottom of the world consists of The Underworld.

After the initial defeat of the Wall of Flesh in The Underworld, the world converts to Hardmode, and new areas of Corruption or Crimson will form, as well as a new biome, The Hallow, in addition to underground versions of those.

The Corruption, Crimson, and Hallow are all "contagious" biomes which spread to certain adjacent blocks. Prior to entering Hardmode, the Evil Biome will only slowly spread with its grass, but once Hardmode begins all three biomes will spread directly through most Soil blocks, Vines, and Thorny bushes.

Space
Space is the layer that appears at the top of the world: at about 700 feet, or more for medium sized worlds. The sky is dark, and even if the sun is still visible, you can see the stars. Low gravity affects players and enemies. Enemies will still spawn in space, but not as often. Space has a unique background and music as well as its own unique enemies - Harpies from the start of the game and Wyverns, Martian Probes and during Hardmode.

Surface/Underground
Biomes that appear above the Cavern layer function as surface biomes, although they can and often extend underground, below 0 feet, in which their distinct underground music will start to play. Actual "underground" versions of biomes, such as the Ice biome, with different subterranean features, do not begin until the depth of the Cavern layer.

Purity / Forest
Forests are the most common surface biome, and are where the player usually spawns upon starting a new world. Green grass grows across the dirt, which can spawn Mushrooms, Dayblooms, and Trees. Forests appear whenever no other biome is valid.

Snow
Snow biomes are winter-themed biomes predominately made of Snow and Ice Blocks. Snow falls constantly, though this is only an aesthetic effect. The Snow biome is always located opposite to the Jungle and thus on the same side of the map as the Dungeon. A minimum of 300 Snow or Ice Blocks are required for an area to count as a Snow biome.

Desert
Deserts are large expanses of Sand that require at least 1000 Sand or other Desert-themed blocks. They are common between Forests.

Corruption and Crimson
A world will always generate with an Evil Biome – either Crimson or Corruption, but never both. Each are hazardous biomes that offer access to important loot, crafting materials, and bosses. On the, the player is able to choose which biome their world is generated with, provided they have defeated the Wall of Flesh on a separate world, but otherwise it is a 50/50 chance of which one is randomly selected during world generation.
 * The Corruption follows a rotten theme and features vertical chasms, Shadow Orbs, and Demon Altars.
 * The Crimson follows a gory theme and features angled passages, Crimson Hearts, and Crimson Altars.

Jungle
The Jungle is a difficult surface biome composed of Jungle Grass, Vines, Mud and Mahogany Trees. The sky appears a vibrant green color. Mahogany Trees respawn automatically without the need to plant Acorns. The Jungle will always be located on the opposite side of the map from the Snow biome and Dungeon.

Dungeon
The Dungeon is a difficult labyrinth leading from the Surface down to near-Underworld depths. The Old Man stands at the entrance, and using him to defeat Skeletron is required to gain access to the inside. Heading below 0ft in the Dungeon without defeating Skeletron spawns one or more Dungeon Guardians, which have enormous defenses and are capable of one-hitting most players. The loot found in the Dungeon is rare, valuable, and some cannot be found elsewhere. The Dungeon is always located opposite to the Jungle and thus on the same side of the map as the Snow biome.

Ocean
Oceans consist of a deep body of water, where dangerous swimming enemies are found, and Water Chests contain ocean-themed loot. They are always located on both horizontal edges of the world.

Duke Fishron can be summoned here with a Truffle Worm.

Mushroom
The surface version of the Glowing Mushroom biome does not occur naturally, and can only be cultivated by players using Mushroom Grass Seeds or an imported Clentaminator with Dark Blue Solution on surface-level mud. It will always appear darker than night, regardless of the time.

Cavern
The Cavern layer marks the beginning of distinct underground biome variants, with separate characteristics from their surface versions.

The Underground Desert is a difficult biome located beneath the surface Desert. The biome always has either a circular or an oval shape and contains various types of Antlions as well as ancient-Egyptian-themed enemies in Hardmode.

Underground Jungle
The Underground Jungle is a difficult biome located beneath the surface Jungle. Its dense catacomb of compartments contain several hazards and dense vegetation, as well as important loot and crafting materials. In Hardmode, valuable Chlorophyte Ore can be mined here. Three bosses can be fought in this biome: An Underground Jungle always contains several Bee Hives, where the Queen Bee boss is usually summoned. After the Mechanical Bosses have been defeated, Plantera's Bulbs spawn, allowing the player to summon Plantera, an important boss. The Jungle Temple is also located within the Underground Jungle, where the Golem boss can be summoned after Plantera is defeated.

Underground Mushroom
The Underground Glowing Mushroom biome features Glowing Mushrooms on top of Mud, and if there is enough height in the cavern, Giant Glowing Mushrooms can grow as well. It has a unique background and is the only place where Truffle Worms can be obtained.

Ice
The Ice biome is the Cavern layer's, more dangerous portion of the Snow biome and is located beneath the surface part. Many ledges and bodies of water will have a layer of Thin Ice over them, and while these can support the player's weight they can break when landing on them with high enough speed. This can be avoided with Ice Skates or Frostspark Boots. Ice Chests contain ice-themed equipment.

Underworld
The Underworld is a difficult biome located along the entire bottom of the world. Lava pools cover much of its terrain and dangerous enemies spawn frequently. Hellstone can be mined here, Ruined Houses offer valuable loot (some within their Shadow Chests), and unique furniture, and the Wall of Flesh boss can be summoned here in order to activate Hardmode. In Hardmode, the spawns in the Underworld, which can be converted to the  NPC using Purification Powder.

Hardmode
Hardmode biomes spawn or convert as a result of the game transitioning to Hardmode.

The Hallow
The Hallow can be considered the counterpart to the Evil Biome. It is automatically created once the player activates Hardmode by defeating the Wall of Flesh. The Hallow has pastel-colored imagery but is nevertheless highly difficult.

Underground Hallow
The Underground Hallow is located beneath the surface Hallow, starting at the Cavern layer. It has a different soundtrack from the surface Hallow, and contains even more difficult enemies as well as valuable items, like Souls of Light and Crystal Shards.

Underground Corruption
The Underground Corruption is created in the Cavern layer upon defeating the Wall of Flesh. It is not necessarily located beneath pre-Hardmode surface Corruption biomes. While it shares the theme with the surface Corruption, it has different enemies and a different soundtrack (a mix of the regular Underground and Corruption soundtrack). It is the only place to obtain Cursed Flames and Souls of Night, which are occasionally dropped by all enemies.

Underground Crimson
The Underground Crimson is the alternative to the Underground Corruption, which functions and generates in mostly the same way. The Underground Crimson, like the Corruption variant, will have enemies occasionally drop Souls of Night. It is the only place where enemies drop Ichor.

Corrupted/Crimson Deserts & Hallowed Deserts
When the world enters Hardmode, several Deserts of the world will become either corrupted, crimsoned, or hallowed. These have their own backgrounds, and spawn unique versions of the Mummy with unique drops. Sand is replaced by Ebonsand, Crimsand, or Pearlsand. It is possible for a pre-Hardmode world to be generated with a Corrupt or Crimson Desert.

Mini-Biomes
Biomes in this category tend to be small in number and size. They have unique characteristics and monster spawns.

Bee Hive
The Bee Hive is located in the Underground Jungle. Bee Hives are made from Hive blocks and contain honey and often a Larva. Destroying the Larva summons the Queen Bee boss. Breaking a Hive block may spawn a bee and/or some honey. There can rarely be two or three Bee Hives next to each other.

The Granite Cave is a cave area made completely of Granite Blocks, usually fairly spacious with interspersed ledges and platforms. Granite Caves contain granite-themed enemies such as Granite Elementals and Granite Golems.

The Graveyard is formed around large clusters of Gravestones due to numerous player deaths. Graveyards generate Ecto Mist and spawn enemies such as Ravens, Ghosts and Maggot Zombies. NPC's who have homes in a Graveyard may sell unique items. Gravestones dropped by newly killed players will take longer to break and cannot be used to form Graveyards.

 Oasis 

The Oasis is a mini-biome the spawns in the Desert. It consists of palm trees surrounding a pool of water in the center.

Jungle Temple
The Jungle Temple is a large, nearly impenetrable structure that spawns deep in the Underground Jungle. It is filled with powerful enemies and is only accessible after defeating Plantera, using the Temple Key she drops to open the door. In the largest chamber, there is a Lihzahrd Altar to spawn Golem.

The Marble Cave is a cave area that is horizontal in shape and made completely out of Marble Blocks. Marble Caves contain ancient-Greek-themed enemies such as Hoplites and Medusas.

Meteorite
When either Eater of Worlds or Brain of Cthulhu has been slain, there is a chance that within the next day the game will spawn a Meteorite and display the message "A meteorite has landed!". The meteor will crash into land and convert the area into a crater with three layers; blocks in the innermost area will be deleted without dropping anything; surrounding this all blocks around the empty area will be converted into Meteorite ore; in a much larger radius outside this mass of ore, only occasional blocks similarly converted. Meteorite ore will afflict the player with the Burning debuff on contact, unless they either have an accessory to nullify its effects, or are under the effect of an Obsidian Skin Potion. Mining Meteorite requires 50% pickaxe power, i.e. a Tungsten Pickaxe, Gold Pickaxe, or better is mandatory. Meteor Heads ( and, in Hardmode, Lava Bats) override all native enemies of the biome the meteor crashed into.

Spider Cave
A Spider Cave is characterized by the black background wall and the numerous webs filling the area. This biome will spawn Wall Creepers in Pre-Hardmode and Black Recluses during Hardmode. The Cobwebs rapidly grow back, which is a unique feature of the biome. Spider Nests are the only location where Web Covered Chests are found, and therefore the only place to obtain a Web Slinger. The Stylist NPC is found in this biome.

Town
A Town is a player-created mini biome that can be made by having 2 or more NPCs living next to each other.

Micro-Biomes
These biomes have unique tiles or structures, but do not have unique monster spawns.

The Campsite is found in the Underground and Cavern layers usually containing a Campfire, Silver Coin Pile, and a background object of skeletal remains. The campsite is 12–20 blocks wide, and not quite as tall.

Gemstone Cave
A Gemstone Cave is filled with concentrated gem deposits of one or more varieties and features a unique background (a gemstone wall) that is speckled with gems of the same color found in the Gemstone Cave.

Moss Chamber
A micro-biome found in the Underground and Cavern layers that contains one color of Moss with a matching back wall.

A micro-biome found in the Ice biome that is made entirely of Thin Ice and an occasional ore vein.

Treasure Rooms
Treasure rooms are spawned upon world creation and appear as if they were man-made structures. They generally contain loot and furniture.

The Enchanted Sword Shrine houses an Enchanted Sword, an Arkhalis, or a fake sword. Every shrine will contain one breakable background sword sprite, which has a 2/3 chance of being a fake sword and a 1/3 chance of being a real sword, which has a 1/10 chance of dropping the Arkhalis, instead of the Enchanted Sword, when destroyed.

Floating Islands
Floating Islands are independent masses of land in the sky. They contain a Skyware Chest with valuable loot.

Jungle Shrine
Jungle Shrines are small brick buildings and typically contain only a single Ivy Chest and a light source. They are found in the Underground Jungle and several can appear in one biome.

Living Tree
Living Trees are huge, thick trees consisting of foreground blocks (as opposed to regular background trees). Cutting the tree yields regular Wood, while the leaves yield nothing. Some Living Trees can be hollow and lead deep underground. There may be rooms containing Living Wood Chests with related loot inside, as well as Living Wood furniture. It is not guaranteed that one will spawn in every world.

Living Mahogany Tree
Living Mahogany Trees are large trees generated in the Underground Jungle. Cutting the tree yields Rich Mahogany wood, while the leaves yield nothing. They always have a hollow trunk, which contains an Ivy Chest.

Pyramid
Pyramids are rarely generated in the Desert upon world creation. They are made of Sandstone Bricks and contain a narrow passage. The treasure room is filled with Coin Stashes, vases and a Gold Chest that can contain unique loot. There is also a passage to a cave at the bottom.

Ruined House
Ruined Houses are generated upon world creation in The Underworld in the middle 50% of the Underworld. They are towers with multiple levels, constructed with missing parts for a "ruined" look, from Obsidian Brick or Hellstone Brick. They are sometimes partially submerged in lava and often contain deposits of Hellstone just outside their walls.

Underground Cabin
Underground Cabins are found in the Cavern layer. They are typically wooden buildings full of dusty, broken furniture as well as chests, vases, and occasionally usable furniture and decorations. As of, Underground Cabins spawn with blocks representing their biome.

Removed Biomes
The following two biomes were removed in the Mobile 1.3.0.7 update.

Heart Shrines were heart-shaped caves sometimes found in Snow biomes. They held two Crystal Hearts and a chest that contained heart-themed items.

A Jungle Sanctum was a structure that occasionally appeared in the Jungle. Jungle Sanctums were larger versions of Jungle Shrines and contained Coin Stashes, along with an Ivy Chest that contained several Jungle-themed items.

Hybrid Biomes
Although many basic biomes appear at the world generation or are triggered by events, hybrid biomes may be synthesized by world mechanics or through player action or in Hardmode.

Hybrid Biomes
Hybrid biomes are those created by the requirements being met for multiple biomes in a certain area. The simplest examples are Hallowed, Crimson, or Corrupt Deserts; however, not only Deserts can be hybridized. Floating Islands can be Corrupted/Crimsoned or Hallowed upon entry of Hardmode, and such high-layered biomes are even required by some Angler quests. Player action can similarly introduce Corruption, Crimson, Hallow, Desert, Jungle, Mushroom, Meteorite, or Snow biomes to any location from Underworld to Sky and likely even Space. This can lead to, for example in an Underworld Crimson, Underworld and Crimson enemies both spawning in the applicable areas.

Biome Existence Requirements
The area used for counting required blocks for the existence of biomes at a location appears to be 50 tiles to each side and approximately 42 below and 44 above, from the location of the player. Note that the "Compatible Biomes" column lists the biomes which can co-exist in the same general location, and has nothing to do with spreading mechanics.

Trivia

 * There were originally supposed to be alternative biomes for Jungle, Underworld and Hallow called Swamp, Hell and Candyland, respectively. The Hell was going to be based on the Hell from the Disney's film . However, these ideas were scrapped as the developers changed direction.