Mini Biomes

The Mini Biomes are biomes small in size. You can find them in all major Biomes.

Missing: The tall un-cutable grass mini biomes in swamp caves

Surface Ruins

Can be found on every island, including village islands. In rare cases, two such ruins will spawn.

Surface ruins will contain one chest (or barrel or some storage container) with some random loot and one of the following: Handgun (with some Simple Bullets), Copper Pitchfork, Nunchucks, Fuzzy Dice, or Fins. In rare cases, village structures or other structures may interfere with a surface ruin spawn.

Underground Ruins

Can be found in all Caves and Deep Caves. Consists of some random furniture and a single chest.
Generally, the treasure will consist of one crop of the appropriate biome (along with seeds) and other random assortments, such as arrows, bombs, potions, coins, etc. Underground ruins chests do not contain a special loot item, unlike Small Treasure Rooms.

Vampire Crypt

Can be found in Forest Caves and Snow Caves.
Crypts will contain Grave Stones that can be mined with a pickaxe for some random items and Stone Coffins that can be looted. (See Treasures.) As the name implies this biome tends to spawn the vampire enemy which can take the form of a bat.

Spider Nest

Can be found in Forest, Snow and Swamp Caves. Spider webs continuously spawn in this biome. Watch out for glowing Eggs that can spawn in the Snow Caves. Breaking one of these will immediately summon the boss, Queen Spider. The biome is also known to spawn the Giant Cave Spider, Black Cave Spider, Small Swamp Cave Spider and Swamp Cave Spider depending on the surface biome.

Small Treasure Room

Small treasure rooms with or without Traps spawn in all biome caves.
Small treasure rooms contain one special loot item of the appropriate biome, plus other random assorted drops. (See Caves for a list of possible items.)

Treasure Room

These treasure rooms are filled with crates and vases full of random goods. (See Treasures.)

Puzzle Treasure Room

Large Room filled with four memory puzzles, two small loot rooms to either side (with auxiliary switches in case another player traps you), and one large loot room unlocked by traversing the puzzle. Found in all Caves. There also exists a version where several traps go off in a pattern. Players must navigate through this maze of traps by timing their movement to the pattern of the traps. At the end is the usual chests and coins.

Thorns

Thorns mini biome in the Swamp Caves.

Ancient Statue Hall

Ancient Statue mini biome in the Desert Caves. It will be filled with crates and vases full of treasures (see Treasures for a list of possible loot), but don't smash the statue unless you are prepared to fight the Ancient Vulture.

Abandoned Castle

A mini biome in the Deep Forest Caves.