World Features

There are two or three interrelated categories which provide various features to a world. Some do simple modifications of the terrain, others add complex structures. Most take either no modifiers, or a single material block which determines what they're made of. Unless otherwise noted, those taking material blocks can accept any solid block.

Up to V11, these were divided into two categories, based on their implementation within the game: "Populators" and "Terrain Alterations". The difference between "terrain alterations" and "populators" is based ultimately on how they work in the code. In general, "populators" run late in chunk generation and add things, while "terrain alterations" run early in chunk generation (approximately right after biome top replacement) and generally remove stone. Ore placement, for example, runs in the populator stage. In code, the distinction is that a "terrain alteration" must be deterministic for a seed and chunk, and must be able to generate a large feature (such as a cave system) section by section without having to "draw" what it is doing into other chunks (such as how a village plops down buildings across multiple chunks).

In V12, these groups were merged and redivided into "Small Features", "Medium Features", and "Large Features". These categories are based on how prominent they are in the world; for example, large but scarce structures are in Medium rather than Large. There are explicit "dummy" entries for each category, but "No Small Features" and "No Medium Features" produce instability (presumably from overcontrolling the world).

Populators
Crystalline Formations Deep Lakes Dense Ores Dungeons Huge Trees Mineshafts Nether Fortress Obelisks Spikes Star Fissure Strongholds Surface Lakes Villages

Terrain Alterations
Caves Floating Islands Ravines Skylands Spheres Tendrils

Small Features

 * Crystalline Formations: These wavelike masses appear near the surface of the age. They have a restricted selection of materials:  Currently, the following materials count as crystalline:  Glass, ice, packed ice, snow, glowstone, nether quartz ore, and Mystcraft's own crystal.


 * Deep Lakes: Produces lakes well below sea level.  Can be made of any available liquid, ice, or packed ice.
 * Lacking Small Features: This "dummy" symbol satisfies the category without adding any particular feature. However, it does add instability to the world.
 * Obelisks: Produces tall towers made of a chosen material.  The towers do not spawn over the void, but can spawn over ocean.
 * Star Fissure: Near the spawn, this will produce a single irregular canyon, penetrating down to and through the bedrock and beyond, and floored with starry portal blocks.  Any entities falling to the bottom, instead of falling into the Void, will be linked to the Home Dimension (normally the Overworld) spawn point.  A Star Fissure provides a guaranteed exit from a world, even if all books are lost.  Takes no modifiers.  Warning:  Occasionally, the portal at the bottom can be blocked by incidental blocks, which can cut off the escape with a lethal impact:  Fallen snow is the most common culprit, but lava and water from the sides of the fissure can also create cobblestone on the bottom.
 * Surface Lakes: Produces lakes on or just under the surface. Can be made of any available liquid, ice, or packed ice.

Medium Features

 * Dungeons: Produces vanilla-style dungeons underground, each with a spawner and loot chests.  Takes no modifiers.
 * Lacking Medium Features: This "dummy" symbol satisfies the category without adding any features.  It does, however, add instability.    Takes no modifiers.
 * Mineshaft: Produces abandoned mineshafts underground, similar to those of the Overworld.  Takes no modifiers.
 * Nether Fortress: Produces Nether Fortresses, including netherwart farms and blaze spawners. Aside from the spawners, mob spawning will be as normal for the world.  For a standard landscape, these are likely to be on the surface.  Takes no modifiers.
 * Ravines: Carves away terrain to make ravines similar to the Overworld.  Takes no modifiers.
 * Spheres: Produces large spheres, of the given material.  These are placed both above, near, and below sea level.  In V12 their frequency was sharply reduced.
 * Spikes: Produces steep, irregular pillars of the given material.
 * Strongholds: The world will have 3 strongholds, placed similarly to the Overworld's.  They can be located with Eyes of Ender, but their End Portals will not work.  No modifiers.
 * Villages: The world will have villages similar to the overworld, but only if it has biomes and terrain where villages can spawn.  No modifiers.

Large Features

 * Caves: Carves branching cave networks out of the terrain.  No modifiers.
 * Dense Ores: Causes extra ores to be placed.  In V10 the symbol added considerable instability, in later versions this is handled indirectly by ore profiling.  No modifiers.
 * Floating Islands: Produces floating islands up around cloud level.  Takes both a material (any solid block including ores), and a biome, which will affect the islands' "decoration" and the area above it.
 * Huge Trees: Adds massive giant trees, vanilla biomes will have appropriate types.  These trees produce normal saplings, and cannot be grown.  No modifiers.
 * Lacking Large Features: This "dummy" symbol satisfies the category without adding anything to the Age.  Unlike the other two of its type, it does not add instability.  No modifiers.
 * Skylands: Carves away most of the terrain, leaving scattered floating islands.  The bottom of the Age will be open to the Void.
 * Tendrils: Generates cave-like structures of a block, rather than carved out with air. Each tendril system will tend to be smaller than a cave system, although some large tendrils may be bigger than some small cave systems. Takes a material (any solid block including ores).