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.
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.
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.
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.)
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.
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.
A mini biome in the Deep Forest Caves.