Overworld Underground Structures
Structures hidden deep beneath the surface, including strongholds, ancient cities, and trial chambers.
Spherical structure with three layers
Massive deep slate city structure
Network of wooden-supported tunnels
End portal room with silverfish spawner
Buried archaeological structure
Copper and tuff block construction
What each underground structure is for
Six of these generate below the surface. Mineshafts and geodes turn up while you mine anyway; the other four are worth a planned trip.
Mineshaft
Long wooden tunnels with rails, and every chest down there sits in a minecart. Thick cobwebs in a side corridor often hide a cave spider spawner, so slow down when you see them. In badlands biomes mineshafts of dark oak generate right on the surface, and since badlands also have extra gold ore higher up, those are an easy stop.
Stronghold
The End portal room is the only room every stronghold is guaranteed to have: twelve portal frames around a pool of lava, plus a silverfish spawner. Each frame has a 10% chance of already holding an eye of ender. Libraries, when there is one, have chests with books and enchanted books. A Java world has 128 strongholds spread over eight rings, and the first ring of three lies between 1,280 and 2,816 blocks from the center of the world.
Ancient City
A huge city with its floor at Y -51, found only in the deep dark. Its chests are the only source of echo shards, disc fragments and the Ward and Silence trim templates, and they can hold Swift Sneak books. The risk is the warden: every natural sculk shrieker you set off raises your warning level, and the fourth shriek summons one.
Trail Ruins
Mostly buried, with just the top of a tower above ground, in taigas, snowy taigas, old growth forests and jungles. They are an archaeology site. Brush the suspicious gravel and the rare finds are pottery sherds, the Relic music disc and the Wayfinder, Raiser, Shaper and Host trim templates. Break that gravel with a shovel and whatever was inside is gone.
Trial Chamber
Copper and tuff halls with an entrance room between Y -40 and Y -20. Trial spawners send waves of mobs, more of them for every player in range, and a cleared spawner has a 50% chance to drop a trial key for everyone. A key opens a vault, and each player can unlock a given vault only once. Breezes spawn nowhere else, so this is where breeze rods come from.
Amethyst Geode
Strictly speaking a feature, not a structure, generating between Y -58 and Y 30 with a shell of smooth basalt and calcite. About 8% of the inner blocks are budding amethyst, the block the crystals grow on. It drops nothing, even with Silk Touch, so leave it in place and come back later to harvest the clusters.
Finding them
Eyes of ender lead to the nearest stronghold, but every throw has a 20% chance of shattering the eye, so bring more than the twelve a portal can need. With cheats, /locate structure takes minecraft:mineshaft (or minecraft:mineshaft_mesa for the badlands kind), minecraft:stronghold, minecraft:ancient_city, minecraft:trail_ruins and minecraft:trial_chambers. Geodes have no ID to locate, because they are not structures.
Staying alive down there
In an ancient city, sneak everywhere. Sneaking stops your movement from making vibrations, and a block of wool between you and a sculk sensor keeps it from picking you up. In a mineshaft, remember that cave spiders fit through gaps a normal spider cannot, and on Normal or Hard their bite poisons you.
In a trial chamber, a friend makes every wave bigger rather than easier. The heavy core for a mace only drops from ominous vaults, at a 7.5% chance. To get there, drink an Ominous Bottle before you go in so the spawners turn ominous; a cleared ominous spawner drops an ominous trial key 30% of the time.