Minecraft World Generation

World Generation

World generation (worldgen/levelgen) places biomes, terrain features, structures, and initial mobs step by step. Java Edition data pack customization is covered by "Custom world generation". Java Edition only unless noted.

Generation Phases

Chunks generate progressively; chunks the player hasn't reached stay at lower phases (proto chunks) until needed, then upgrade to level chunks. Phases:

1. Empty — nothing generated.
2. Structure Starts — no blocks yet; computes which structures can place here, their ranges, pieces, and start points.
3. Structure References — computes referenced pieces so structures aren't lost/truncated; persisted in chunk data.
4. Biomes — computes all biome cells; persisted. Still no blocks.
5. Noise — density functions fill initial blocks, aquifers place fluids, veins generate. First phase with blocks.
6. Surface — surface rules cover the terrain; bedrock and deepslate layers generate here.
7. Carvers — carve terrain, replacing blocks with air.
8. Features — structures and features place in fixed order; heightmaps complete (decoration).
9. Initialize Light — registers light sources.
10. Light — computes sky/block light propagation.
11. Spawn — initial animal spawns.
12. Full — converts the proto chunk to a level chunk; applies deferred block updates.

Randomization

• PRNGs: the Java standard LCG (only 48 bits used) and Xoroshiro128++ (uses seeds more fully; similar seeds give dissimilar results). Same seed → same world.
• Noise: mostly Perlin noise (gradient-filled via PRNG for seed dependence, interpolated for smoothness), plus simplex noise and combinations. All game noise uses fractal stacking, and two similar noises are combined for variety.

Biome Generation

Biomes are checked/placed/saved per biome cell of 4×4×4 blocks. For finer operations (rendering, per-position carving) the biome is "magnified" to each block by picking the nearest cell (distance + position-dependent random).

Biome sources:

• Checkerboard — square-ish biomes repeating along diagonals.
• Fixed — one biome everywhere.
• The End (Java) — five biomes: The End (within ~1024 blocks of (0,0)), Small End Islands, End Barrens, End Midlands, End Highlands placed by erosion density gradient.
• Multi-noise — used by the Overworld and the Nether by default.

Multi-noise uses 6 density functions: temperature, humidity/vegetation, continentalness (low = ocean, very low = mushroom fields), erosion (low = rugged/mountains, high = flat), weirdness (variants, e.g. sunflower plains), and depth (which height a biome occupies). All 6 are visible in the debug screen's "NoiseRouter" line.

Placement: for each biome cell, compute the 6 density values; the chosen biome is the one minimizing b² + Σ distance(mᵢ,nᵢ,vᵢ)² over all placement intervals [mᵢ,nᵢ] (offset b; distance = 0 inside, else distance to the nearer bound).

• Overworld uses all 6 functions with offsets 0. Levels: temperature −2.22..2.22 in 5 bands; humidity −1.69..1.69 in 5 bands; continentalness −3.66..3.66 in 7 bands (mushroom fields < −1.05, deep ocean, ocean, coast, near inland, inland, far inland > 0.3); erosion −2.42..2.42 in 7 bands. Ridginess p = 1 − |3|w| − 2| from weirdness w, in 5 bands (rivers < −0.85, lowlands, low hills, high hills, peaks > 0.7). Placement proceeds: compute depth (rises 0.03125 per 4-block cell downward) → ocean vs land by continentalness → land subdivided by ridginess/erosion/continentalness → beach/badlands/low hills/plateaus/windswept classes refined by temperature/humidity/weirdness. (Caves in depth 0.2–0.9 can overlap and confuse /locate biome.) Full tables: see the Minecraft Wiki "World generation" page or worldgen/ JSON files in the source.
• Nether uses only temperature, humidity, and offsets (other four functions fixed 0).

Initial Terrain

The terrain generator decides air/fluid/vein/default block per position.

Final Density

A position gets the dimension's default block iff the final density function n(x,y,z) > 0; otherwise air or fluid.

Aquifers

Aquifers handle all fluids (and air) where the final density ≤ 0. First the preliminary surface level is computed: scan down from the top until the initial density (without jaggedness, debug "AS") > 0.390625.

Per-position fluid status + fluid surface level:

1. Base selector: below −54 and below sea level → lava at level −54 (hardcoded); else dimension default fluid at sea level.
2. More than 20 blocks above the preliminary surface → use the base selector result.
3. Compare with helper positions' preliminary surface levels (min s); under specific conditions borrow a nearby position's fluid status (8+ blocks below sea level cases).
4. Final fluid state: below −54 and below sea level → lava; ≥ −10 → default fluid; in between → lava noise (per 64×40×64 region, |noise| > 0.3 → lava).
5. Final fluid level: eroded (e < −0.225) & deep (d > 0.9) → none; otherwise clamp flood noise f to −1..1, compute thresholds from s (mapped 1→0 over 0..64), flood threshold maps to −0.3..0.8, fluid threshold to −0.8..0.4; f > flood threshold → base selector level; f > fluid threshold → level = min(40⌊h/140⌋ + 3⌊hf/10⌋ + 20, s) using spread noise (16×40×16 regions); else none.

Placement (per position): lava directly; else split world into 16×12×16 regions, sample 12 candidate regions (offset +1 Y, −5 XZ), find the 3 nearest sample positions with squared distances; similarity s = 1 − 0.04|d1−d2|; pressure function from fluid levels + barrier noise; combine similarities × pressure with the final density to decide default block vs fluid vs air (block updates scheduled as flagged).

If aquifers are disabled, the fluid status is used directly.

Veins

Placed after aquifers (skipped where aquifers placed air/fluid). Controlled by vein toggle T, vein ridged R, and vein gap G densities:

1. T > 0 → copper vein, else iron vein (hardcoded).
2. Copper only at Y 0–50; iron at Y −60–−8.
3. Height clamp check: need |T| + min(0.01·min(hmax−h, h−hmin) − 0.2, 0) < 0.4 to place (copper: |T| + (−0.005(|h−20|+|h−30|) + 0.05); iron: |T| + (−0.005(|h+40|+|h+28|) + 0.06)).
4. 30% of vein blocks randomly removed (random > 0.7 → skip).
5. R ≥ 0 → skip; 6. G ≤ −0.3 → vein filler block (granite for copper, tuff for iron); 7. random ≥ min(|T|−0.3, 0.3) → filler; else 2% raw metal block, 98% ore. (≥70% filler, ≤0.6% raw metal blocks.)

Overworld Initial Terrain

Aquifers and veins enabled; sea level 63; default block stone; default fluid water.

• Terrain splines (depend only on continentalness c, erosion e, weirdness w → ridginess pv): terrain offset spline (the depth density's Y=128 slice; controls heights — oceans slightly below sea level, deep oceans lower, mushroom fields raised above water), terrain factor spline (vertical stretching), and jaggedness (height offsets combined with jagged noise; not in the initial density). Low erosion → rugged/high; high erosion → flat/low; high ridginess → peaks; river-level ridginess → valleys slightly below sea level; extreme weirdness can carve ultra-deep pits with lava.
• Noise caves — four components: cheese caves (un-abs'd noise → round bubbles), spaghetti caves (abs'd noise near 0 → winding tunnels), noodle caves (same but thin; density fixed 64 in some spots), noise pillars (pillar frequency × thickness noise). Final density: sloped_cheese (split at 1.5625: surface vs underground) → surface: caves/entrances; underground: min(entrances, cheese, noodle) + max with pillars; density fixed toward −0.078125 above 240 (256) and +0.1171875 below −40 (−64) to cap terrain and protect bedrock; finally min with noodle caves (which ignore the deep clamp, so deep underground favors noodles).

Nether Initial Terrain

Aquifers and veins disabled; sea level 32; default block netherrack; default fluid lava (all air pockets below 32 fill with lava). Final density is a "smeared" noise; fixed toward 0.9375 at Y 104–128 and 2.5 at Y −8–24 to cap the bedrock layers; terrain max height 128.

End Initial Terrain

Aquifers/veins disabled; sea level 32; default block end stone; default fluid air. Smeared noise + End erosion density (erodes terrain so the main island is separate from outer islands); fixed toward −23.4375 at Y 56–312 and −0.234375 at Y 4–32 — the lowest initial block is Y 4 and high terrain is rare.

Surface Rules

After initial terrain, surface rules replace blocks per biome (grass, sand, etc.) and also place bedrock/deepslate. The surface rule context contains: biome, surface noise, surface depth (⌊2.75s + 0.25r + 3⌋), surface secondary noise, water height (1 above nearest fluid surface; differs when invoked by carvers), stone depth above (non-fluid blocks to nearest air above), stone depth below, and min surface level (interpolated neighbor-chunk terrain min + surface depth − 8; at/above = surface).

Scanning: per horizontal position at the WORLD_SURFACE_WG heightmap; the eroded badlands rule (hardcoded, un-disableable, badlands only) places terrain pillars when badlandsSurfaceNoise and badlandsPillarNoise are both > 0, height from badlandsPillarRoofNoise (skips water); then surface rules apply top-down; the frozen ocean rule (hardcoded, frozen ocean/deep frozen ocean only) builds icebergs (snow cap, packed ice body, water inside) when icebergSurfaceNoise and icebergPillarNoise > 1.8, height from icebergPillarRoofNoise, melting by biome temperature.

Surface rule conditions (first match wins; no match keeps the initial block): above/below/at height y; height gradient b→t (probability ramps); surface; biome; noise n (Y=0 slice); cold/warm (biome snow temperature); basin (surface depth < 0); steep slope (≥4 block height difference east–west or north–south on WORLD_SURFACE_WG); underwater n blocks / not underwater (carver-context differs); floor / ceiling (with n-block depth variants); max/min surface level (± surface depth); nearest surface height (nearest air above − 1). Some conditions are affected by surface secondary noise (marked * on the wiki). The Overworld's rule table is on the wiki tutorial page; the Nether's likewise; the End has a single rule: everything → end stone.

Terrain Carving

Carvers run after surface rules. The game checks 17×17 chunks around the current chunk; chunks marked as a carver's start chunk (random, configurable probability) carve the current chunk. Carvers: cave carvers (up to 15 loop iterations, 25% chance of a ring chamber per iteration; 1–4 tunnels otherwise, tunnels may fork if wide enough) and canyon carvers (giant fissure of ellipsoid carvings with platforms at some heights). Carving per position: skip if already carved (caving mask) or not replaceable; compute carved state — below carver lava level → lava; else aquifer function at final density 0 decides lava/water/air (no carve if it returns default block); if the pre-carve block was grass/mycelium with dirt below, re-run surface rules on the dirt (carver context: stone depths 1, water height = position height — the reason water checks differ from surface phase).

• Overworld: cave carver (start 15%, Y −56..180), cave carver (start 7%, Y −56..47), canyon carver (start 1%, Y 10..67); all replace #overworld_carver_replaceables.
• Nether: one cave carver (start 20%, Y 0..126), replaces #nether_carver_replaceables.

Structure Generation

Structure placement works per structure set (a group of structures). Two placement types:

• Random spread — spacing (grid cell size; at most one start per cell) and separation (exclusion ring east/south). E.g. spacing 27, separation 4: cells where x%27 or z%27 ≥ 23 can never host a start.
• Concentric rings — ring width 2.5d chunks, gaps 3.5d, first ring 2.75d from origin (d = distance multiplier); per-ring count caps c1 = s (spread), cₙ = cₙ₋₁ + ⌊2cₙ₋₁/n⌋; starts placed evenly by random start angle (random distance within the ring); preferred-biome sets try to shift placement.

Then frequency filters starts (spacing out dense ones) and exclusion zones keep structures away from specified structures. A chunk at the Structure Starts phase checks all structure sets: if it's a start chunk, pick a structure from the set, find a placeable position (biome + height requirements); if none, try the next structure; if at least one fits, create the structure start, compute all pieces, and derive the bounding box. At Structure References, each chunk collects all structures in 17×17 chunks around it and stores references for intersecting bounding boxes (avoids duplicating data). Consequence: a structure's horizontal span cannot exceed 17 chunks — starts and farthest pieces beyond 8 chunks get truncated.

Overworld structure sets and their distributions (random spread vs concentric rings), Nether sets (all random spread), and End sets (all random spread): see the Minecraft Wiki "World generation" page for full tables.

Feature and Structure Placement

Decoration has 11 stages (structure pieces always place before features within the same stage; order otherwise by definition):

1. Raw Generation — raw terrain features (only End floating islands).
2. Lakes — lava lakes only (water lakes were replaced by aquifers).
3. Local Modifications — icebergs, mossy boulders, large dripstone, basalt pillars, amethyst geodes.
4. Underground Structures — fossils, dungeons.
5. Surface Structures — desert wells, deltas, end gateways, blue ice, ice spikes.
6. Strongholds — does nothing (strongholds are not placed here).
7. Underground Ores — ores and disks.
8. Underground Decoration — non-ore clusters (incl. Nether ore clusters), sculk patches.
9. Fluid Springs — springs.
10. Vegetal Decoration — vegetation: trees, bamboo, random patches.
11. Top Layer Modification — frozen top layer only.

Structure pieces are placed strictly within the current chunk (parts outside are truncated and placed by other chunks); pieces keep their data for later placement. Features may modify the 3×3 chunk area around the generating chunk — this makes feature placement order-dependent: two chunks generating in different orders can produce different results with the same seed (and concurrent generation may interleave blocks). Player paths influence which order wins.

Lighting

After decoration: Initialize Light registers light sources without propagation (prevents truncated edge lighting), then Light propagates sky and block light (max level 15; neighboring chunks at least reached Initialize, so correctness is guaranteed).

Initial Entity Spawn

Java Edition. After lighting, the chunk spawns an initial animal (category "animal"): chance 7% in snowy plains/ice spikes, 10% elsewhere; a random animal from the biome's spawns; per-chunk count computed; up to 4 wandering attempts (each attempt re-snaps to the highest block position, staying inside the chunk); basic spawn rules apply (no collision). Mob caps are ignored. Some entities (e.g. structure-attached) are placed with structure pieces instead.